diff --git a/lib/json0.js b/lib/json0.js
index dc3a405..a93ffe4 100644
--- a/lib/json0.js
+++ b/lib/json0.js
@@ -43,6 +43,16 @@ var clone = function(o) {
   return JSON.parse(JSON.stringify(o));
 };
 
+var validateListIndex = function(key) {
+  if (typeof key !== 'number')
+    throw new Error('List index must be a number');
+};
+
+var validateObjectKey = function (key) {
+  if (typeof key !== 'string')
+    throw new Error('Object key must be a number');
+};
+
 /**
  * JSON OT Type
  * @type {*}
@@ -162,6 +172,12 @@ json.apply = function(snapshot, op) {
       elem = elem[key];
       key = p;
 
+      if (isArray(elem) && typeof key !== 'number')
+        throw new Error('List index must be a number');
+
+      if (isObject(elem) && typeof key !== 'string')
+        throw new Error('Object key must be a string');
+
       if (parent == null)
         throw new Error('Path invalid');
     }
@@ -175,6 +191,9 @@ json.apply = function(snapshot, op) {
       if (typeof elem[key] != 'number')
         throw new Error('Referenced element not a number');
 
+      if (typeof c.na !== 'number')
+        throw new Error('Number addition is not a number');
+
       elem[key] += c.na;
     }
 
@@ -200,6 +219,9 @@ json.apply = function(snapshot, op) {
 
     // List move
     else if (c.lm !== void 0) {
+      if (typeof c.lm !== 'number')
+        throw new Error('List move target index must be a number');
+
       json.checkList(elem);
       if (c.lm != key) {
         var e = elem[key];
diff --git a/lib/text0.js b/lib/text0.js
index e26c6a9..238d448 100644
--- a/lib/text0.js
+++ b/lib/text0.js
@@ -60,6 +60,10 @@ var checkValidOp = function(op) {
 text.apply = function(snapshot, op) {
   var deleted;
 
+  var type = typeof snapshot;
+  if (type !== 'string')
+    throw new Error('text0 operations cannot be applied to type: ' + type);
+
   checkValidOp(op);
   for (var i = 0; i < op.length; i++) {
     var component = op[i];
diff --git a/test/json0.coffee b/test/json0.coffee
index 531f76e..e2ee6df 100644
--- a/test/json0.coffee
+++ b/test/json0.coffee
@@ -64,6 +64,11 @@ genTests = (type) ->
       c_s = type.apply leftHas, right_
       assert.deepEqual s_c, c_s
 
+    it 'throws when adding a string to a number', ->
+      assert.throws -> type.apply 1, [{p: [], na: 'a'}]
+
+    it 'throws when adding a number to a string', ->
+      assert.throws -> type.apply 'a', [{p: [], na: 1}]
 
   # Strings should be handled internally by the text type. We'll just do some basic sanity checks here.
   describe 'string', ->
@@ -72,6 +77,12 @@ genTests = (type) ->
       assert.deepEqual 'bc', type.apply 'abc', [{p:[0], sd:'a'}]
       assert.deepEqual {x:'abc'}, type.apply {x:'a'}, [{p:['x', 1], si:'bc'}]
 
+      it 'throws when the target is not a string', ->
+        assert.throws -> type.apply [1], [{p: [0], si: 'a'}]
+
+      it 'throws when the inserted content is not a string', ->
+        assert.throws -> type.apply 'a', [{p: [0], si: 1}]
+
     describe '#transform()', ->
       it 'splits deletes', ->
         assert.deepEqual type.transform([{p:[0], sd:'ab'}], [{p:[1], si:'x'}], 'left'), [{p:[0], sd:'a'}, {p:[1], sd:'b'}]
@@ -127,6 +138,24 @@ genTests = (type) ->
         assert.deepEqual ['a', 'b', 'c'], type.apply ['b', 'a', 'c'], [{p:[1], lm:0}]
         assert.deepEqual ['a', 'b', 'c'], type.apply ['b', 'a', 'c'], [{p:[0], lm:1}]
 
+      it 'throws when keying a list replacement with a string', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p: ['0'], li: 'x', ld: 'a'}]
+
+      it 'throws when keying a list insertion with a string', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p: ['0'], li: 'x'}]
+
+      it 'throws when keying a list deletion with a string', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p: ['0'], ld: 'a'}]
+
+      it 'throws when keying a list move with a string', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p: ['0'], lm: 0}]
+
+      it 'throws when specifying a string as a list move target', ->
+        assert.throws -> type.apply ['a', 'b', 'c'], [{p: [1], lm: '0'}]
+
+      it 'throws when an array index part-way through the path is a string', ->
+        assert.throws -> type.apply {arr:[{x:'a'}]}, [{p:['arr', '0', 'x'], od: 'a'}]
+
       ###
       'null moves compose to nops', ->
         assert.deepEqual [], type.compose [], [{p:[3],lm:3}]
@@ -389,6 +418,15 @@ genTests = (type) ->
       assert.deepEqual [], type.transform [{p:['k'], od:'x'}], [{p:['k'], od:'x'}], 'left'
       assert.deepEqual [], type.transform [{p:['k'], od:'x'}], [{p:['k'], od:'x'}], 'right'
 
+    it 'throws when the insertion key is a number', ->
+      assert.throws -> type.apply {'1':'a'}, [{p:[2], oi: 'a'}]
+
+    it 'throws when the deletion key is a number', ->
+      assert.throws -> type.apply {'1':'a'}, [{p:[1], od: 'a'}]
+
+    it 'throws when an object key part-way through the path is a number', ->
+      assert.throws -> type.apply {'1': {x: 'a'}}, [{p:[1, 'x'], od: 'a'}]
+
   describe 'randomizer', ->
     @timeout 20000
     @slow 6000