diff --git a/core/block.js b/core/block.js
index 162d87368..a00cc911d 100644
--- a/core/block.js
+++ b/core/block.js
@@ -1635,25 +1635,21 @@ Blockly.Block.prototype.interpolate_ = function(message, args, lastDummyAlign,
   // An array of [field, fieldName] tuples.
   var fieldStack = [];
   for (var i = 0, element; (element = elements[i]); i++) {
-    switch (element['type']) {
-      case 'input_value':
-      case 'input_statement':
-      case 'input_dummy':
-        var input = this.inputFromJson_(element, warningPrefix);
-        // Should never be null, but just in case.
-        if (input) {
-          for (var j = 0, tuple; (tuple = fieldStack[j]); j++) {
-            input.appendField(tuple[0], tuple[1]);
-          }
-          fieldStack.length = 0;
+    if (this.isInputKeyword_(element['type'])) {
+      var input = this.inputFromJson_(element, warningPrefix);
+      // Should never be null, but just in case.
+      if (input) {
+        for (var j = 0, tuple; (tuple = fieldStack[j]); j++) {
+          input.appendField(tuple[0], tuple[1]);
         }
-        break;
+        fieldStack.length = 0;
+      }
+    } else {
       // All other types, including ones starting with 'input_' get routed here.
-      default:
-        var field = this.fieldFromJson_(element);
-        if (field) {
-          fieldStack.push([field, element['name']]);
-        }
+      var field = this.fieldFromJson_(element);
+      if (field) {
+        fieldStack.push([field, element['name']]);
+      }
     }
   }
 };
@@ -1722,10 +1718,7 @@ Blockly.Block.prototype.interpolateArguments_ =
       }
 
       var length = elements.length;
-      var startsWith = Blockly.utils.string.startsWith;
-      // TODO: This matches the old behavior, but it doesn't work for fields
-      //   that don't start with 'field_'.
-      if (length && startsWith(elements[length - 1]['type'], 'field_')) {
+      if (length && !this.isInputKeyword_(elements[length - 1]['type'])) {
         var dummyInput = {'type': 'input_dummy'};
         if (lastDummyAlign) {
           dummyInput['align'] = lastDummyAlign;
@@ -1808,6 +1801,19 @@ Blockly.Block.prototype.inputFromJson_ = function(element, warningPrefix) {
   return input;
 };
 
+/**
+ * Returns true if the given string matches one of the input keywords.
+ * @param {string} str The string to check.
+ * @return {boolean} True if the given string matches one of the input keywords,
+ *     false otherwise.
+ * @private
+ */
+Blockly.Block.prototype.isInputKeyword_ = function(str) {
+  return str == 'input_value' ||
+      str == 'input_statement' ||
+      str == 'input_dummy';
+};
+
 /**
  * Turns a string into the JSON definition of a label field. If the string
  * becomes an empty string when trimmed, this returns null.
diff --git a/core/input.js b/core/input.js
index 253160a64..9a165226e 100644
--- a/core/input.js
+++ b/core/input.js
@@ -14,7 +14,7 @@ goog.provide('Blockly.Input');
 
 goog.require('Blockly.Connection');
 goog.require('Blockly.constants');
-goog.require('Blockly.FieldLabel');
+goog.require('Blockly.fieldRegistry');
 
 
 /**
@@ -100,7 +100,10 @@ Blockly.Input.prototype.insertFieldAt = function(index, field, opt_name) {
 
   // Generate a FieldLabel when given a plain text field.
   if (typeof field == 'string') {
-    field = new Blockly.FieldLabel(/** @type {string} */ (field));
+    field = /** @type {!Blockly.Field} **/ (Blockly.fieldRegistry.fromJson({
+      'type': 'field_label',
+      'text': field,
+    }));
   }
 
   field.setSourceBlock(this.sourceBlock_);
diff --git a/tests/mocha/block_json_test.js b/tests/mocha/block_json_test.js
index ed5301116..6b93339fd 100644
--- a/tests/mocha/block_json_test.js
+++ b/tests/mocha/block_json_test.js
@@ -69,6 +69,7 @@ suite('Block JSON initialization', function() {
           type: 'test',
           interpolateArguments_: Blockly.Block.prototype.interpolateArguments_,
           stringToFieldJson_: Blockly.Block.prototype.stringToFieldJson_,
+          isInputKeyword_: Blockly.Block.prototype.isInputKeyword_,
         };
         chai.assert.deepEqual(
             block.interpolateArguments_(tokens, args, lastAlign),
@@ -217,7 +218,7 @@ suite('Block JSON initialization', function() {
           ]);
     });
 
-    test.skip('Add last dummy for no_field_prefix_field', function() {
+    test('Add last dummy for no_field_prefix_field', function() {
       this.assertInterpolation(
           [
             {
@@ -236,6 +237,25 @@ suite('Block JSON initialization', function() {
           ]);
     });
 
+    test('Add last dummy for input_prefix_field', function() {
+      this.assertInterpolation(
+          [
+            {
+              'type': 'input_prefix_field',
+            }
+          ],
+          [],
+          undefined,
+          [
+            {
+              'type': 'input_prefix_field',
+            },
+            {
+              'type': 'input_dummy',
+            }
+          ]);
+    });
+
     test('Set last dummy alignment', function() {
       this.assertInterpolation(
           ['test1', 'test2', 'test3'],
diff --git a/tests/mocha/input_test.js b/tests/mocha/input_test.js
index c135bfe97..e88a84062 100644
--- a/tests/mocha/input_test.js
+++ b/tests/mocha/input_test.js
@@ -57,6 +57,16 @@ suite('Inputs', function() {
         this.dummy.insertFieldAt(0, 'field');
         chai.assert.instanceOf(this.dummy.fieldRow[0], Blockly.FieldLabel);
       });
+      test('String w/ field_label overwritten', function() {
+        Blockly.fieldRegistry.unregister('field_label');
+        Blockly.fieldRegistry.register('field_label', Blockly.FieldNumber);
+
+        this.dummy.insertFieldAt(0, '1');
+        chai.assert.instanceOf(this.dummy.fieldRow[0], Blockly.FieldNumber);
+
+        Blockly.fieldRegistry.unregister('field_label');
+        Blockly.fieldRegistry.register('field_label', Blockly.FieldLabel);
+      });
       test('Empty String', function() {
         this.dummy.insertFieldAt(0, '');
         chai.assert.isEmpty(this.dummy.fieldRow);