diff --git a/src/compiler/jsgen.js b/src/compiler/jsgen.js
index 757f0fca..8ccdcd91 100644
--- a/src/compiler/jsgen.js
+++ b/src/compiler/jsgen.js
@@ -893,6 +893,9 @@ class JSGenerator {
             }
             if (procedureData.yields) {
                 this.source += 'yield* ';
+                if (!this.script.yields) {
+                    throw new Error('Script uses yielding procedure but is not marked as yielding.');
+                }
             }
             this.source += `thread.procedures["${sanitize(procedureCode)}"](`;
             // Only include arguments if the procedure accepts any.