diff --git a/src/core/display/chunks/chunkFactory.js b/src/core/display/chunks/chunkFactory.js
index 93e1600c..d868afe2 100644
--- a/src/core/display/chunks/chunkFactory.js
+++ b/src/core/display/chunks/chunkFactory.js
@@ -119,7 +119,7 @@ SceneJS_ChunkFactory.prototype.putChunk = function (chunk) {
             chunk.recycle();
         }
 
-        this._chunks[chunk.id] = null;
+        delete this._chunks[chunk.id];
 
         var freeChunks = SceneJS_ChunkFactory._freeChunks[chunk.type];
 
@@ -140,7 +140,7 @@ SceneJS_ChunkFactory.prototype.webglRestored = function () {
 
             chunk = this._chunks[chunkId]; // Re-cache chunk's shader variable locations
 
-            if (chunk.build) {
+            if (chunk && chunk.build) {
                 chunk.build();
             }
         }
diff --git a/src/core/display/programFactory.js b/src/core/display/programFactory.js
index da9625df..5f676285 100644
--- a/src/core/display/programFactory.js
+++ b/src/core/display/programFactory.js
@@ -44,7 +44,7 @@ SceneJS_ProgramFactory.prototype.putProgram = function(program) {
 
         SceneJS_ProgramSourceFactory.putSource(program.hash);
 
-        this._programs[program.hash] = null;
+       delete this._programs[program.hash];
     }
 };
 
@@ -54,10 +54,14 @@ SceneJS_ProgramFactory.prototype.putProgram = function(program) {
 SceneJS_ProgramFactory.prototype.webglRestored = function() {
 
     var gl = this._canvas.gl;
+    var program;
 
     for (var id in this._programs) {
         if (this._programs.hasOwnProperty(id)) {
-            this._programs[id].build(gl);
+            program = this._programs[id];
+            if (program && program.build) {
+                program.build(gl);
+            }
         }
     }
 };
diff --git a/src/core/scene/coreFactory.js b/src/core/scene/coreFactory.js
index e5c2dbc3..3d961f72 100644
--- a/src/core/scene/coreFactory.js
+++ b/src/core/scene/coreFactory.js
@@ -135,7 +135,7 @@ SceneJS_CoreFactory.prototype.putCore = function (core) {
 
         var cores = this._cores[core.type];
 
-        cores[core.coreId] = null;
+        delete cores[core.coreId];
 
         this._stateMap.removeItem(core.stateId);  // Release state ID for reuse
     }
@@ -161,7 +161,7 @@ SceneJS_CoreFactory.prototype.webglRestored = function () {
 
                         core = cores[coreId];
 
-                        if (core.webglRestored) { // Method augmented on core by user
+                        if (core && core.webglRestored) { // Method augmented on core by user
                             core.webglRestored();
                         }
                     }