diff --git a/core/field_dropdown.ts b/core/field_dropdown.ts
index 7c86653b3..cff223045 100644
--- a/core/field_dropdown.ts
+++ b/core/field_dropdown.ts
@@ -125,6 +125,8 @@ export class FieldDropdown extends Field {
 
     if (Array.isArray(menuGenerator)) {
       validateOptions(menuGenerator);
+      // Deep copy the option structure so it doesn't change.
+      menuGenerator = JSON.parse(JSON.stringify(menuGenerator));
     }
 
     /**
diff --git a/tests/mocha/json_test.js b/tests/mocha/json_test.js
index 383b49130..a0905c576 100644
--- a/tests/mocha/json_test.js
+++ b/tests/mocha/json_test.js
@@ -275,7 +275,7 @@ suite('JSON Block Definitions', function() {
 
       const image1 = options[1][0];
       assertImageEquals(IMAGE1, image1);
-      chai.assert.equal(IMAGE1.alt, IMAGE1_ALT_TEXT);  // Via Msg reference
+      chai.assert.equal(image1.alt, IMAGE1_ALT_TEXT);  // Via Msg reference
       chai.assert.equal(VALUE1, options[1][1]);
 
       const image2 = options[2][0];