diff --git a/src/blocks/scratch3_procedures.js b/src/blocks/scratch3_procedures.js
index c648d9603..b91630f12 100644
--- a/src/blocks/scratch3_procedures.js
+++ b/src/blocks/scratch3_procedures.js
@@ -27,11 +27,20 @@ class Scratch3ProcedureBlocks {
         if (!util.stackFrame.executed) {
             const procedureCode = args.mutation.proccode;
             const paramNames = util.getProcedureParamNames(procedureCode);
+
+            // If null, procedure could not be found, which can happen if custom
+            // block is dragged between sprites without the definition.
+            // Match Scratch 2.0 behavior and noop.
+            if (paramNames === null) {
+                return;
+            }
+
             for (let i = 0; i < paramNames.length; i++) {
                 if (args.hasOwnProperty(`input${i}`)) {
                     util.pushParam(paramNames[i], args[`input${i}`]);
                 }
             }
+
             util.stackFrame.executed = true;
             util.startProcedure(procedureCode);
         }