diff --git a/src/blocks/scratch3_procedures.js b/src/blocks/scratch3_procedures.js
index 56d3f3e7c..35ed6921d 100644
--- a/src/blocks/scratch3_procedures.js
+++ b/src/blocks/scratch3_procedures.js
@@ -13,17 +13,16 @@ class Scratch3ProcedureBlocks {
      */
     getPrimitives () {
         return {
-            procedures_definition: this.definition,
+            // procedures_definition is the top block of a procedure but has no
+            // effect of its own.
+            procedures_definition: null,
+
             procedures_call: this.call,
             argument_reporter_string_number: this.argumentReporterStringNumber,
             argument_reporter_boolean: this.argumentReporterBoolean
         };
     }
 
-    definition () {
-        // No-op: execute the blocks.
-    }
-
     call (args, util) {
         if (!util.stackFrame.executed) {
             const procedureCode = args.mutation.proccode;
diff --git a/src/engine/execute.js b/src/engine/execute.js
index c8638d01a..7c5bc1ef8 100644
--- a/src/engine/execute.js
+++ b/src/engine/execute.js
@@ -279,7 +279,7 @@ class BlockCached {
         // Assign opcode isHat and blockFunction data to avoid dynamic lookups.
         this._isHat = runtime.getIsHat(opcode);
         this._blockFunction = runtime.getOpcodeFunction(opcode);
-        this._definedBlockFunction = typeof this._blockFunction !== 'undefined';
+        this._definedBlockFunction = typeof this._blockFunction === 'function';
 
         // Store the current shadow value if there is a shadow value.
         const fieldKeys = Object.keys(fields);
diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 3979b415e..47a4c75af 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -713,7 +713,7 @@ class Runtime extends EventEmitter {
                 if (packageObject.getPrimitives) {
                     const packagePrimitives = packageObject.getPrimitives();
                     for (const op in packagePrimitives) {
-                        if (packagePrimitives.hasOwnProperty(op)) {
+                        if (typeof packagePrimitives[op] === 'function') {
                             this._primitives[op] =
                                 packagePrimitives[op].bind(packageObject);
                         }