diff --git a/src/engine/blocks.js b/src/engine/blocks.js
index e22122ed..db0d5a5b 100644
--- a/src/engine/blocks.js
+++ b/src/engine/blocks.js
@@ -735,6 +735,12 @@ class Blocks {
                 if (this._blocks[e.newParent].inputs.hasOwnProperty(e.newInput)) {
                     oldShadow = this._blocks[e.newParent].inputs[e.newInput].shadow;
                 }
+
+                // If the block being attached is itself a shadow, make sure to set
+                // both block and shadow to that blocks ID. This happens when adding
+                // inputs to a custom procedure.
+                if (this._blocks[e.id].shadow) oldShadow = e.id;
+
                 this._blocks[e.newParent].inputs[e.newInput] = {
                     name: e.newInput,
                     block: e.id,
diff --git a/test/unit/engine_blocks.js b/test/unit/engine_blocks.js
index c25d5b42..8ab483e2 100644
--- a/test/unit/engine_blocks.js
+++ b/test/unit/engine_blocks.js
@@ -358,6 +358,43 @@ test('move no obscure shadow', t => {
     t.end();
 });
 
+test('move - attaching new shadow', t => {
+    const b = new Blocks(new Runtime());
+    // Block/shadow are null to mimic state right after a procedure_call block
+    // is mutated by adding an input. The "move" will attach the new shadow.
+    b.createBlock({
+        id: 'foo',
+        opcode: 'TEST_BLOCK',
+        next: null,
+        fields: {},
+        inputs: {
+            fooInput: {
+                name: 'fooInput',
+                block: null,
+                shadow: null
+            }
+        },
+        topLevel: true
+    });
+    b.createBlock({
+        id: 'bar',
+        opcode: 'TEST_BLOCK',
+        shadow: true,
+        next: null,
+        fields: {},
+        inputs: {},
+        topLevel: true
+    });
+    b.moveBlock({
+        id: 'bar',
+        newInput: 'fooInput',
+        newParent: 'foo'
+    });
+    t.equal(b._blocks.foo.inputs.fooInput.block, 'bar');
+    t.equal(b._blocks.foo.inputs.fooInput.shadow, 'bar');
+    t.end();
+});
+
 test('change', t => {
     const b = new Blocks(new Runtime());
     b.createBlock({