diff --git a/src/blocks/scratch3_procedures.js b/src/blocks/scratch3_procedures.js
index 5caea72ee..86aacd059 100644
--- a/src/blocks/scratch3_procedures.js
+++ b/src/blocks/scratch3_procedures.js
@@ -54,7 +54,9 @@ class Scratch3ProcedureBlocks {
     argumentReporterStringNumber (args, util) {
         const value = util.getParam(args.VALUE);
         if (value === null) {
-            return '';
+            // When the parameter is not found in the most recent procedure
+            // call, the default is always 0.
+            return 0;
         }
         return value;
     }
@@ -62,7 +64,9 @@ class Scratch3ProcedureBlocks {
     argumentReporterBoolean (args, util) {
         const value = util.getParam(args.VALUE);
         if (value === null) {
-            return false;
+            // When the parameter is not found in the most recent procedure
+            // call, the default is always 0.
+            return 0;
         }
         return value;
     }
diff --git a/src/engine/thread.js b/src/engine/thread.js
index 316642222..47777a97a 100644
--- a/src/engine/thread.js
+++ b/src/engine/thread.js
@@ -349,6 +349,7 @@ class Thread {
             if (frame.params.hasOwnProperty(paramName)) {
                 return frame.params[paramName];
             }
+            return null;
         }
         return null;
     }