KeyNotFoundException that makes no sense (*I* can find the key)

I don't get it, what am I doing wrong? Or why is the compiler suddenly being stupid? I've used Dictionaries of all different types before without any trouble .

 var letterToWords : Dictionary.<int[], List.< GameObject > >;


 function RemoveLetter (where : int[]) { // does the deleted letter destroy one or more words, or form new words?
     Debug.Log ("where:" + "["+where[0]+","+where[1]+","+where[2]+"]");
     Debug.Log ("letterToWords.Count: " +letterToWords.Count);
     var debugKeys : List.<int[]> = letterToWords.Keys.ToList();
     var n : int = debugKeys.Count;
     while (n--) {
         Debug.Log ("key "+n+": ["+debugKeys[n][0]+","+debugKeys[n][1]+","+debugKeys[n][2]+"]");
         }
     Debug.Log ("letterToWords [where]: " +letterToWords[where]);

and here is the output...

 where:[71,0,85]
 
 letterToWords.Count: 11
 
 key 10: [71,0,92]
 
 key 9: [71,0,91]
 
 key 8: [71,0,90]
 
 key 7: [71,0,89]
 
 key 6: [71,0,88]
 
 key 5: [71,0,87]
 
 key 4: [71,0,86]
 
 key 3: [71,0,85] <---look! there it is! right there!
 
 key 2: [71,0,84]
 
 key 1: [71,0,83]
 
 key 0: [71,0,82]
 
 KeyNotFoundException: The given key was not present in the dictionary.
 
 System.Collections.Generic.Dictionary`2[System.Int32[],System.Collections.Generic.List`1[UnityEngine.GameObject]].get_Item (System.Int32[] key) (at /Users/builduser/buildslave/monoAndRuntimeClassLibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150)
 WordMatrix.RemoveLetter (System.Int32[] where) (at Assets/_Scripts/Base/WordMatrix.js:331)
 LetterMatrix.Update () (at Assets/_Scripts/Base/LetterMatrix.js:150)

I'm stumped. I also put in debug messages where the key-value pair is assigned, and it seems to be working right on that end...

 lm.SelectCell (dx,dy,dz);
 var sC : int[] = new int[3];
 sC[0] = dx;
 sC[1] = dy;
 sC[2] = dz;
 //Debug.Log ("letterList.Add("+sC[0]+","+sC[1]+","+sC[2]);
 letterList.Add (sC);
 if (letterToWords.ContainsKey (sC) ) {
     if (!letterToWords[sC].Contains (wordObj)) {
         letterToWords[sC].Add (wordObj);
         }
     }
 else {
     var ltwl : List.<GameObject> = new List.<GameObject>();
     ltwl.Add (wordObj);
     letterToWords.Add (sC,ltwl);
     }    
 lm.affectedCells.Add (sC);
 lm.ChangeCell (dx,dy,dz, wordToPut[l]);
 lm.ChangeNeighbors();
 lm.BakeMeshes();
 Debug.Log ("letterToWords.Count" + letterToWords.Count);
 Debug.Log ("letterToWords[" +sC[0]+","+sC[1]+","+sC[2]+"][0]:" + letterToWords[sC][0]);
 }

and get this output to the console, as I would expect :

...

 letterToWords.Count4
 letterToWords[71,0,85][0]:ABRACADABRA (UnityEngine.GameObject)

...