diff --git a/app/assets/javascripts/workers/worker_world.js b/app/assets/javascripts/workers/worker_world.js
index a1c4baf0f..3dcae25d2 100644
--- a/app/assets/javascripts/workers/worker_world.js
+++ b/app/assets/javascripts/workers/worker_world.js
@@ -289,6 +289,7 @@ self.setupDebugWorldToRunUntilFrame = function (args) {
             return;
         }
         Math.random = self.debugWorld.rand.randf;  // so user code is predictable
+        Aether.replaceBuiltin("Math", Math);
     }
     self.debugWorld.totalFrames = args.frame; //hack to work around error checking
     self.currentDebugWorldFrame = args.frame;
@@ -343,6 +344,7 @@ self.runWorld = function runWorld(args) {
     return;
   }
   Math.random = self.world.rand.randf;  // so user code is predictable
+  Aether.replaceBuiltin("Math", Math);
   self.postMessage({type: 'start-load-frames'});
   self.world.loadFrames(self.onWorldLoaded, self.onWorldError, self.onWorldLoadProgress);
 };
diff --git a/app/lib/Angel.coffee b/app/lib/Angel.coffee
index fbdb252db..aae10c724 100644
--- a/app/lib/Angel.coffee
+++ b/app/lib/Angel.coffee
@@ -211,6 +211,7 @@ module.exports = class Angel extends CocoClass
   doSimulateWorld: (work) ->
     work.t1 = now()
     Math.random = work.testWorld.rand.randf  # so user code is predictable
+    Aether.replaceBuiltin("Math", Math)
     i = 0
     while i < work.testWorld.totalFrames
       frame = work.testWorld.getFrame i++
diff --git a/app/models/ThangType.coffee b/app/models/ThangType.coffee
index e1ccf1054..84fd8e713 100644
--- a/app/models/ThangType.coffee
+++ b/app/models/ThangType.coffee
@@ -159,7 +159,7 @@ module.exports = class ThangType extends CocoModel
     spriteSheet = @builder.build()
     @logBuild @t0, false, @options.portraitOnly
     @spriteSheets[key] = spriteSheet
-    delete @building[key]
+    @building[key] = false
     @builder = null
     @options = null
     spriteSheet
@@ -171,7 +171,7 @@ module.exports = class ThangType extends CocoModel
     buildQueue[0].t0 = new Date().getTime() if buildQueue[0]
     buildQueue[0]?.buildAsync()
     @spriteSheets[key] = e.target.spriteSheet
-    delete @building[key]
+    @building[key] = false
     @trigger 'build-complete', {key:key, thangType:@}
     @vectorParser = null
 
diff --git a/app/schemas/models/user.coffee b/app/schemas/models/user.coffee
index a6525bf91..5b4a93ca7 100644
--- a/app/schemas/models/user.coffee
+++ b/app/schemas/models/user.coffee
@@ -60,7 +60,7 @@ UserSchema = c.object {},
     colorConfig: c.object {additionalProperties: c.colorConfig()}
 
   aceConfig: c.object {},
-    language: {type: 'string', 'default': 'javascript', 'enum': ['javascript', 'coffeescript', 'clojure', 'lua', 'python', 'io']}
+    language: {type: 'string', 'default': 'javascript', 'enum': ['javascript', 'coffeescript', 'python', 'clojure', 'lua', 'io']}
     keyBindings: {type: 'string', 'default': 'default', 'enum': ['default', 'vim', 'emacs']}
     invisibles: {type: 'boolean', 'default': false}
     indentGuides: {type: 'boolean', 'default': false}
diff --git a/app/views/play/level/tome/spell_view.coffee b/app/views/play/level/tome/spell_view.coffee
index 04851959e..9933033d0 100644
--- a/app/views/play/level/tome/spell_view.coffee
+++ b/app/views/play/level/tome/spell_view.coffee
@@ -18,9 +18,9 @@ module.exports = class SpellView extends View
   editModes:
     'javascript': 'ace/mode/javascript'
     'coffeescript': 'ace/mode/coffee'
+    'python': 'ace/mode/python'
     'clojure': 'ace/mode/clojure'
     'lua': 'ace/mode/lua'
-    'python': 'ace/mode/python'
     'io': 'ace/mode/text'
 
   keyBindings:
diff --git a/headless_client/worker_world.coffee b/headless_client/worker_world.coffee
index e40317bfe..667481858 100644
--- a/headless_client/worker_world.coffee
+++ b/headless_client/worker_world.coffee
@@ -91,6 +91,7 @@ work = () ->
       self.onWorldError error
       return
     Math.random = self.world.rand.randf # so user code is predictable
+    Aether.replaceBuiltin("Math", Math)
     console.log "Loading frames."
 
     self.postMessage type: "start-load-frames"