diff --git a/core/field_variable.js b/core/field_variable.js
index f93caa150..242cee9f0 100644
--- a/core/field_variable.js
+++ b/core/field_variable.js
@@ -29,6 +29,7 @@ goog.provide('Blockly.FieldVariable');
 goog.require('Blockly.FieldDropdown');
 goog.require('Blockly.Msg');
 goog.require('Blockly.Variables');
+goog.require('goog.asserts');
 goog.require('goog.string');
 
 
@@ -73,6 +74,16 @@ Blockly.FieldVariable.prototype.init = function() {
   }
 };
 
+/**
+ * Attach this field to a block.
+ * @param {!Blockly.Block} block The block containing this field.
+ */
+Blockly.FieldVariable.prototype.setSourceBlock = function(block) {
+  goog.asserts.assert(!block.isShadow(),
+      'Variable fields are not allowed to exist on shadow blocks.');
+  Blockly.FieldVariable.superClass_.setSourceBlock.call(this, block);
+};
+
 /**
  * Get the variable's name (use a variableDB to convert into a real name).
  * Unline a regular dropdown, variables are literal and have no neutral value.
diff --git a/core/xml.js b/core/xml.js
index 2567560cd..8ee0e24af 100644
--- a/core/xml.js
+++ b/core/xml.js
@@ -535,6 +535,9 @@ Blockly.Xml.domToBlockHeadless_ = function(xmlBlock, workspace) {
       goog.asserts.assert(child.isShadow(),
                           'Shadow block not allowed non-shadow child.');
     }
+    // Ensure this block doesn't have any variable inputs.
+    goog.asserts.assert(block.getVars().length == 0,
+        'Shadow blocks cannot have variable fields.');
     block.setShadow(true);
   }
   return block;