diff --git a/pico/sek.c b/pico/sek.c
index bd8f70f5..ccbddb8e 100644
--- a/pico/sek.c
+++ b/pico/sek.c
@@ -398,7 +398,7 @@ int SekRegisterIdlePatch(unsigned int pc, int oldop, int newop, void *ctx)
 
   // XXX: probably shouldn't patch RAM too
   v = m68k_read16_map[pc >> M68K_MEM_SHIFT];
-  if (!(v & 0x80000000))
+  if (~v & ~((uptr)-1LL >> 1)) // MSB clear?
     target = (u16 *)((v << 1) + pc);
   else {
     if (++idledet_bads > 128)