diff --git a/tools/push3-transpiler/src/transpiler.ts b/tools/push3-transpiler/src/transpiler.ts
index 5aae3a3..d6da0de 100644
--- a/tools/push3-transpiler/src/transpiler.ts
+++ b/tools/push3-transpiler/src/transpiler.ts
@@ -31,15 +31,13 @@ function emit(state: TranspilerState, line: string): void {
 
 function dpop(state: TranspilerState, ctx: string): string {
   const v = state.dStack.pop();
-  // Stack underflow → Push3 no-op semantics: treat missing value as 0
-  if (v === undefined) return '0';
+  if (v === undefined) throw new Error(`DYADIC stack underflow at ${ctx}`);
   return v;
 }
 
 function bpop(state: TranspilerState, ctx: string): string {
   const v = state.bStack.pop();
-  // Stack underflow → Push3 no-op semantics: treat missing bool as false
-  if (v === undefined) return 'false';
+  if (v === undefined) throw new Error(`BOOLEAN stack underflow at ${ctx}`);
   return v;
 }