diff --git a/src/lj_def.h b/src/lj_def.h
index e458c89f..5e63da3e 100644
--- a/src/lj_def.h
+++ b/src/lj_def.h
@@ -104,9 +104,10 @@ typedef unsigned int uintptr_t;
 #define checku16(x)	((x) == (int32_t)(uint16_t)(x))
 #define checki32(x)	((x) == (int32_t)(x))
 #define checku32(x)	((x) == (uint32_t)(x))
+#define checkptr31(x)	(((uint64_t)(uintptr_t)(x) >> 31) == 0)
 #define checkptr32(x)	((uintptr_t)(x) == (uint32_t)(uintptr_t)(x))
 #define checkptr47(x)	(((uint64_t)(uintptr_t)(x) >> 47) == 0)
-#define checkptrGC(x)	(LJ_GC64 ? checkptr47((x)) : LJ_64 ? checkptr32((x)) :1)
+#define checkptrGC(x)	(LJ_GC64 ? checkptr47((x)) : LJ_64 ? checkptr31((x)) :1)
 
 /* Every half-decent C compiler transforms this into a rotate instruction. */
 #define lj_rol(x, n)	(((x)<<(n)) | ((x)>>(-(int)(n)&(8*sizeof(x)-1))))