diff -r bbf9f3809d55 -r e718b56aba78 src/Interpreter.cpp
--- a/src/Interpreter.cpp	Sun Aug 12 20:51:31 2012 -0700
+++ b/src/Interpreter.cpp	Sat Sep 01 16:36:20 2012 -0700
@@ -320,6 +320,10 @@
 
     for (;;) {
         Opcode op = Opcode(*top.pc);
+
+        if (OpcodeInfo[op].nuses > 0)
+            assert(top.sp - OpcodeInfo[op].nuses >= top.fp->stackBase());
+
         switch (op) {
 
 case OP_NOP:
@@ -737,7 +741,7 @@
 case OP_SETARG:
 {
     unsigned slot = READ_UINT32(top.pc);
-    top.fp->arg(slot) = *--top.sp;
+    top.fp->arg(slot) = top.sp[-1];
     break;
 }
 
@@ -1103,6 +1107,7 @@
         }
 
         top.pc += OpcodeInfo[op].length;
+        assert(top.sp >= top.fp->stackBase());
     }
 
 done: