diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index 5b74cf55b..f25827b39 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -152,10 +152,12 @@ class Blocks {
         for (const id in this._blocks) {
             if (!this._blocks.hasOwnProperty(id)) continue;
             const block = this._blocks[id];
-            if ((block.opcode === 'procedures_defnoreturn' ||
-                block.opcode === 'procedures_defreturn') &&
-                this._blocks[block.inputs.custom_block.block].mutation.proccode === name) {
-                return id;
+            if (block.opcode === 'procedures_defnoreturn' ||
+                block.opcode === 'procedures_defreturn') {
+                const internal = this._getCustomBlockInternal(block);
+                if (internal && internal.mutation.proccode === name) {
+                    return id; // The outer define block id
+                }
             }
         }
         return null;
@@ -546,6 +548,17 @@ class Blocks {
         return params;
     }
 
+    /**
+     * Helper to get the corresponding internal procedure definition block
+     * @param {!object} defineBlock Outer define block.
+     * @return {!object} internal definition block which has the mutation.
+     */
+    _getCustomBlockInternal (defineBlock) {
+        if (defineBlock.inputs && defineBlock.inputs.custom_block) {
+            return this._blocks[defineBlock.inputs.custom_block.block];
+        }
+    }
+
     /**
      * Helper to add a stack to `this._scripts`.
      * @param {?string} topBlockId ID of block that starts the script.
diff --git a/test/unit/engine_sequencer.js b/test/unit/engine_sequencer.js
index e5dcd0be7..76323e67c 100644
--- a/test/unit/engine_sequencer.js
+++ b/test/unit/engine_sequencer.js
@@ -153,9 +153,19 @@ test('stepToProcedure', t => {
     t.strictEquals(th.peekStack(), expectedBlock);
     s.stepToProcedure(th, 'faceCode');
     t.strictEquals(th.peekStack(), expectedBlock);
-    s.stepToProcedure(th, 'faceCode');
-    th.target.blocks.getBlock(th.stack[th.stack.length - 4]).mutation.proccode = 'othercode';
+
+    th.target.blocks.createBlock({
+        id: 'internalId',
+        opcode: 'procedures_defnoreturn_internal',
+        mutation: {
+            proccode: 'othercode'
+        }
+    });
     expectedBlock = th.stack[th.stack.length - 4];
+    th.target.blocks.getBlock(expectedBlock).inputs.custom_block = {
+        type: 'custom_block',
+        block: 'internalId'
+    };
     s.stepToProcedure(th, 'othercode');
     t.strictEquals(th.peekStack(), expectedBlock);