diff --git a/libbf.h b/libbf.h
index 48e9d95..0457c18 100644
--- a/libbf.h
+++ b/libbf.h
@@ -27,7 +27,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#if INTPTR_MAX >= INT64_MAX
+#if defined(__SIZEOF_INT128__) && (INTPTR_MAX >= INT64_MAX)
 #define LIMB_LOG2_BITS 6
 #else
 #define LIMB_LOG2_BITS 5