diff --git a/HISTORY.md b/HISTORY.md
index 410b41b9..2733b91b 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,11 +6,12 @@ http://visjs.org
 ### General
 
 - Fixed #893, #911: the `clickToUse` option of Network, Graph2d, and Network 
-  was blocking click events in the web page.  
+  was blocking click events in the web page.
 
 ### Timeline
 
 - Improved function `fit()` to take into account the actual width of items.
+- Fixed #897: Timeline option `{snap: null}` did give a validation error.
 
 ### Graph2D
 
diff --git a/lib/network/options.js b/lib/network/options.js
index cf8c69f0..fcaf7fc5 100644
--- a/lib/network/options.js
+++ b/lib/network/options.js
@@ -11,18 +11,16 @@ let number = 'number';
 let array = 'array';
 let object = 'object'; // should only be in a __type__ property
 let dom = 'dom';
-let fn = 'function';
-let undef = 'undefined';
-let any = 'any'
+let any = 'any';
 
 
 let allOptions = {
   configure: {
     enabled: {boolean},
-    filter: {boolean,string,array,fn},
+    filter: {boolean,string,array,'function': 'function'},
     container: {dom},
     showButton: {boolean},
-    __type__: {object,boolean,string,array,fn}
+    __type__: {object,boolean,string,array,'function': 'function'}
   },
   edges: {
     arrows: {
@@ -51,9 +49,9 @@ let allOptions = {
       __type__: {object,string}
     },
     hidden: {boolean},
-    hoverWidth: {fn,number},
-    label: {string,undef},
-    length: {number,undef},
+    hoverWidth: {'function': 'function',number},
+    label: {string,'undefined': 'undefined'},
+    length: {number,'undefined': 'undefined'},
     physics: {boolean},
     scaling: {
       min: {number},
@@ -66,10 +64,10 @@ let allOptions = {
         drawThreshold: {number},
         __type__: {object,boolean}
       },
-      customScalingFunction: {fn},
+      customScalingFunction: {'function': 'function'},
       __type__: {object}
     },
-    selectionWidth: {fn,number},
+    selectionWidth: {'function': 'function',number},
     selfReferenceSize: {number},
     shadow: {
       enabled: {boolean},
@@ -84,9 +82,9 @@ let allOptions = {
       roundness: {number},
       __type__: {object,boolean}
     },
-    title: {string, undef},
+    title: {string, 'undefined': 'undefined'},
     width: {number},
-    value: {number, undef},
+    value: {number, 'undefined': 'undefined'},
     __type__: {object}
   },
   groups: {
@@ -116,7 +114,7 @@ let allOptions = {
     __type__: {object}
   },
   layout: {
-    randomSeed: {undef,number},
+    randomSeed: {'undefined': 'undefined',number},
     hierarchical: {
       enabled: {boolean},
       levelSeparation: {number},
@@ -129,19 +127,19 @@ let allOptions = {
   manipulation: {
     enabled: {boolean},
     initiallyActive: {boolean},
-    addNode: {boolean,fn},
-    addEdge: {boolean,fn},
-    editNode: {fn},
-    editEdge: {boolean,fn},
-    deleteNode: {boolean,fn},
-    deleteEdge: {boolean,fn},
+    addNode: {boolean,'function': 'function'},
+    addEdge: {boolean,'function': 'function'},
+    editNode: {'function': 'function'},
+    editEdge: {boolean,'function': 'function'},
+    deleteNode: {boolean,'function': 'function'},
+    deleteEdge: {boolean,'function': 'function'},
     controlNodeStyle: 'get from nodes, will be overwritten below',
     __type__: {object,boolean}
   },
   nodes: {
     borderWidth: {number},
-    borderWidthSelected: {number,undef},
-    brokenImage: {string,undef},
+    borderWidthSelected: {number,'undefined': 'undefined'},
+    brokenImage: {string,'undefined': 'undefined'},
     color: {
       border: {string},
       background: {string},
@@ -171,7 +169,7 @@ let allOptions = {
       strokeColor: {string},
       __type__: {object,string}
     },
-    group: {string,number,undef},
+    group: {string,number,'undefined': 'undefined'},
     hidden: {boolean},
     icon: {
       face: {string},
@@ -181,9 +179,9 @@ let allOptions = {
       __type__: {object}
     },
     id: {string, number},
-    image: {string,undef}, // --> URL
-    label: {string,undef},
-    level: {number,undef},
+    image: {string,'undefined': 'undefined'}, // --> URL
+    label: {string,'undefined': 'undefined'},
+    level: {number,'undefined': 'undefined'},
     mass: {number},
     physics: {boolean},
     scaling: {
@@ -197,7 +195,7 @@ let allOptions = {
         drawThreshold: {number},
         __type__: {object, boolean}
       },
-      customScalingFunction: {fn},
+      customScalingFunction: {'function': 'function'},
       __type__: {object}
     },
     shadow: {
@@ -209,8 +207,8 @@ let allOptions = {
     },
     shape: {string:['ellipse', 'circle', 'database', 'box', 'text','image', 'circularImage','diamond', 'dot', 'star', 'triangle', 'triangleDown', 'square','icon']},
     size: {number},
-    title: {string,undef},
-    value: {number,undef},
+    title: {string,'undefined': 'undefined'},
+    value: {number,'undefined': 'undefined'},
     x: {number},
     y: {number},
     __type__: {object}
diff --git a/lib/shared/Validator.js b/lib/shared/Validator.js
index 2af6889b..f9d79670 100644
--- a/lib/shared/Validator.js
+++ b/lib/shared/Validator.js
@@ -84,6 +84,7 @@ class Validator {
   static checkFields(option, options, referenceOptions, referenceOption, refOptionObj, path) {
     let optionType = Validator.getType(options[option]);
     let refOptionType = refOptionObj[optionType];
+    console.log('checkFields', optionType, refOptionType, refOptionObj)
     if (refOptionType !== undefined) {
       // if the type is correct, we check if it is supposed to be one of a few select values
       if (Validator.getType(refOptionType) === 'array') {
@@ -102,18 +103,10 @@ class Validator {
         Validator.parse(options[option], referenceOptions[referenceOption], path);
       }
     }
-    else {
-      if (refOptionObj['undef'] !== undefined && optionType === 'undefined') {
-        // item is undefined, which is allowed
-      }
-      else if (refOptionObj['fn'] !== undefined && optionType === 'function') {
-        // item is a function, which is allowed
-      }
-      else if (refOptionObj['any'] === undefined) {
-        // type of the field is incorrect and the field cannot be any
-        console.log('%cInvalid type received for "' + option + '". Expected: ' + Validator.print(Object.keys(refOptionObj)) + '. Received [' + optionType + '] "' + options[option] + '"' + Validator.printLocation(path, option), printStyle);
-        errorFound = true;
-      }
+    else if (refOptionObj['any'] === undefined) {
+      // type of the field is incorrect and the field cannot be any
+      console.log('%cInvalid type received for "' + option + '". Expected: ' + Validator.print(Object.keys(refOptionObj)) + '. Received [' + optionType + '] "' + options[option] + '"' + Validator.printLocation(path, option), printStyle);
+      errorFound = true;
     }
   }
 
diff --git a/lib/timeline/optionsGraph2d.js b/lib/timeline/optionsGraph2d.js
index 2f73fc99..28ecec72 100644
--- a/lib/timeline/optionsGraph2d.js
+++ b/lib/timeline/optionsGraph2d.js
@@ -13,18 +13,15 @@ let date = 'date';
 let object = 'object'; // should only be in a __type__ property
 let dom = 'dom';
 let moment = 'moment';
-let fn = 'function';
-let nada = 'null';
-let undef = 'undefined';
 let any = 'any';
 
 
 let allOptions = {
   configure: {
     enabled: {boolean},
-    filter: {boolean,fn},
+    filter: {boolean,'function': 'function'},
     container: {dom},
-    __type__: {object,boolean,fn}
+    __type__: {object,boolean,'function': 'function'}
   },
 
   //globals :
@@ -67,13 +64,13 @@ let allOptions = {
     alignZeros: {boolean},
     left:{
       range: {min:{number},max:{number},__type__: {object}},
-      format: {fn},
+      format: {'function': 'function'},
       title: {text:{string,number},style:{string},__type__: {object}},
       __type__: {object}
     },
     right:{
       range: {min:{number},max:{number},__type__: {object}},
-      format: {fn},
+      format: {'function': 'function'},
       title: {text:{string,number},style:{string},__type__: {object}},
       __type__: {object}
     },
@@ -104,25 +101,25 @@ let allOptions = {
   end: {number, date, string, moment},
   format: {
     minorLabels: {
-      millisecond: {string,undef},
-      second: {string,undef},
-      minute: {string,undef},
-      hour: {string,undef},
-      weekday: {string,undef},
-      day: {string,undef},
-      month: {string,undef},
-      year: {string,undef},
+      millisecond: {string,'undefined': 'undefined'},
+      second: {string,'undefined': 'undefined'},
+      minute: {string,'undefined': 'undefined'},
+      hour: {string,'undefined': 'undefined'},
+      weekday: {string,'undefined': 'undefined'},
+      day: {string,'undefined': 'undefined'},
+      month: {string,'undefined': 'undefined'},
+      year: {string,'undefined': 'undefined'},
       __type__: {object}
     },
     majorLabels: {
-      millisecond: {string,undef},
-      second: {string,undef},
-      minute: {string,undef},
-      hour: {string,undef},
-      weekday: {string,undef},
-      day: {string,undef},
-      month: {string,undef},
-      year: {string,undef},
+      millisecond: {string,'undefined': 'undefined'},
+      second: {string,'undefined': 'undefined'},
+      minute: {string,'undefined': 'undefined'},
+      hour: {string,'undefined': 'undefined'},
+      weekday: {string,'undefined': 'undefined'},
+      day: {string,'undefined': 'undefined'},
+      month: {string,'undefined': 'undefined'},
+      year: {string,'undefined': 'undefined'},
       __type__: {object}
     },
     __type__: {object}
@@ -146,8 +143,8 @@ let allOptions = {
   showMinorLabels: {boolean},
   start: {date, number, string, moment},
   timeAxis: {
-    scale: {string,undef},
-    step: {number,undef},
+    scale: {string,'undefined': 'undefined'},
+    step: {number,'undefined': 'undefined'},
     __type__: {object}
   },
   width: {string, number},
@@ -190,12 +187,12 @@ let configureOptions = {
       visible: true,
       alignZeros: true,
       left:{
-        //range: {min:undefined,max:undefined},
+        //range: {min:'undefined': 'undefined'ined,max:'undefined': 'undefined'ined},
         //format: function (value) {return value;},
         title: {text:'',style:''}
       },
       right:{
-        //range: {min:undefined,max:undefined},
+        //range: {min:'undefined': 'undefined'ined,max:'undefined': 'undefined'ined},
         //format: function (value) {return value;},
         title: {text:'',style:''}
       }
diff --git a/lib/timeline/optionsTimeline.js b/lib/timeline/optionsTimeline.js
index 413403e0..7082e8a5 100644
--- a/lib/timeline/optionsTimeline.js
+++ b/lib/timeline/optionsTimeline.js
@@ -13,18 +13,15 @@ let date = 'date';
 let object = 'object'; // should only be in a __type__ property
 let dom = 'dom';
 let moment = 'moment';
-let fn = 'function';
-let nada = 'null';
-let undef = 'undefined';
 let any = 'any';
 
 
 let allOptions = {
   configure: {
     enabled: {boolean},
-    filter: {boolean,fn},
+    filter: {boolean,'function': 'function'},
     container: {dom},
-    __type__: {object,boolean,fn}
+    __type__: {object,boolean,'function': 'function'}
   },
 
   //globals :
@@ -33,39 +30,39 @@ let allOptions = {
   clickToUse: {boolean},
   dataAttributes: {string, array},
   editable: {
-    add: {boolean, undef},
-    remove: {boolean, undef},
-    updateGroup: {boolean, undef},
-    updateTime: {boolean, undef},
+    add: {boolean, 'undefined': 'undefined'},
+    remove: {boolean, 'undefined': 'undefined'},
+    updateGroup: {boolean, 'undefined': 'undefined'},
+    updateTime: {boolean, 'undefined': 'undefined'},
     __type__: {boolean, object}
   },
   end: {number, date, string, moment},
   format: {
     minorLabels: {
-      millisecond: {string,undef},
-      second: {string,undef},
-      minute: {string,undef},
-      hour: {string,undef},
-      weekday: {string,undef},
-      day: {string,undef},
-      month: {string,undef},
-      year: {string,undef},
+      millisecond: {string,'undefined': 'undefined'},
+      second: {string,'undefined': 'undefined'},
+      minute: {string,'undefined': 'undefined'},
+      hour: {string,'undefined': 'undefined'},
+      weekday: {string,'undefined': 'undefined'},
+      day: {string,'undefined': 'undefined'},
+      month: {string,'undefined': 'undefined'},
+      year: {string,'undefined': 'undefined'},
       __type__: {object}
     },
     majorLabels: {
-      millisecond: {string,undef},
-      second: {string,undef},
-      minute: {string,undef},
-      hour: {string,undef},
-      weekday: {string,undef},
-      day: {string,undef},
-      month: {string,undef},
-      year: {string,undef},
+      millisecond: {string,'undefined': 'undefined'},
+      second: {string,'undefined': 'undefined'},
+      minute: {string,'undefined': 'undefined'},
+      hour: {string,'undefined': 'undefined'},
+      weekday: {string,'undefined': 'undefined'},
+      day: {string,'undefined': 'undefined'},
+      month: {string,'undefined': 'undefined'},
+      year: {string,'undefined': 'undefined'},
       __type__: {object}
     },
     __type__: {object}
   },
-  groupOrder: {string, fn},
+  groupOrder: {string, 'function': 'function'},
   height: {string, number},
   hiddenDates: {object, array},
   locale:{string},
@@ -76,8 +73,8 @@ let allOptions = {
   margin: {
     axis: {number},
     item: {
-      horizontal: {number,undef},
-      vertical: {number,undef},
+      horizontal: {number,'undefined': 'undefined'},
+      vertical: {number,'undefined': 'undefined'},
       __type__: {object,number}
     },
     __type__: {object,number}
@@ -88,15 +85,15 @@ let allOptions = {
   minHeight: {number, string},
   moveable: {boolean},
   multiselect: {boolean},
-  onAdd: {fn},
-  onUpdate: {fn},
-  onMove: {fn},
-  onMoving: {fn},
-  onRemove: {fn},
-  order: {fn},
+  onAdd: {'function': 'function'},
+  onUpdate: {'function': 'function'},
+  onMove: {'function': 'function'},
+  onMoving: {'function': 'function'},
+  onRemove: {'function': 'function'},
+  order: {'function': 'function'},
   orientation: {
-    axis: {string,undef},
-    item: {string,undef},
+    axis: {string,'undefined': 'undefined'},
+    item: {string,'undefined': 'undefined'},
     __type__: {string, object}
   },
   selectable: {boolean},
@@ -104,12 +101,12 @@ let allOptions = {
   showMajorLabels: {boolean},
   showMinorLabels: {boolean},
   stack: {boolean},
-  snap: {fn, nada},
+  snap: {'function': 'function', 'null': 'null'},
   start: {date, number, string, moment},
-  template: {fn},
+  template: {'function': 'function'},
   timeAxis: {
-    scale: {string,undef},
-    step: {number,undef},
+    scale: {string,'undefined': 'undefined'},
+    step: {number,'undefined': 'undefined'},
     __type__: {object}
   },
   type: {string},
@@ -157,7 +154,7 @@ let configureOptions = {
       }
     },
 
-    //groupOrder: {string, fn},
+    //groupOrder: {string, 'function': 'function'},
     height: '',
     //hiddenDates: {object, array},
     locale: '',
@@ -174,12 +171,12 @@ let configureOptions = {
     minHeight: '',
     moveable: false,
     multiselect: false,
-    //onAdd: {fn},
-    //onUpdate: {fn},
-    //onMove: {fn},
-    //onMoving: {fn},
-    //onRename: {fn},
-    //order: {fn},
+    //onAdd: {'function': 'function'},
+    //onUpdate: {'function': 'function'},
+    //onMove: {'function': 'function'},
+    //onMoving: {'function': 'function'},
+    //onRename: {'function': 'function'},
+    //order: {'function': 'function'},
     orientation: {
       axis: ['both', 'bottom', 'top'],
       item: ['bottom', 'top']
@@ -189,9 +186,9 @@ let configureOptions = {
     showMajorLabels: true,
     showMinorLabels: true,
     stack: true,
-    //snap: {fn, nada},
+    //snap: {'function': 'function', nada},
     start: '',
-    //template: {fn},
+    //template: {'function': 'function'},
     //timeAxis: {
     //  scale: ['millisecond', 'second', 'minute', 'hour', 'weekday', 'day', 'month', 'year'],
     //  step: [1, 1, 10, 1]
diff --git a/test/timeline.html b/test/timeline.html
index 1f8d2816..0158b433 100644
--- a/test/timeline.html
+++ b/test/timeline.html
@@ -162,6 +162,7 @@
         'hour':       'dddd D MMMM'
       }
     },
+    snap: null,
 //    timeAxis: {
 //      scale: 'hour',
 //      step: 2