@@ -100,8 +100,8 @@ uxn_eval(Uxn *u, Uint16 pc)

 err:

 	/* set 1 in errcode if it involved the return stack instead of the working stack */

-	/*        (stack overflow & (       instruction opcode was STH or JSR        )) ^ Return Mode */

-	errcode |= ((errcode >> 1 & instr >> 1 & instr >> 2 & instr >> 3 & ~instr >> 4) ^ instr >> 6) & 1;

+	/*        (stack overflow & (opcode was STH / JSR)) ^ Return Mode */

+	errcode |= ((errcode >> 1 & (instr & 0x1e) == 0x0e) ^ instr >> 6) & 1;

 	return uxn_halt(u, errcode, pc - 1);

 }