diff --git a/iseq.h b/iseq.h
index 1ca77a34d9..4c6d9f6597 100644
--- a/iseq.h
+++ b/iseq.h
@@ -21,7 +21,7 @@ RUBY_EXTERN const int ruby_api_version[];
 #define ISEQ_MBITS_BITLENGTH (ISEQ_MBITS_SIZE * CHAR_BIT)
 #define ISEQ_MBITS_SET(buf, i) (buf[(i) / ISEQ_MBITS_BITLENGTH] |= ((iseq_bits_t)1 << ((i) % ISEQ_MBITS_BITLENGTH)))
 #define ISEQ_MBITS_SET_P(buf, i) ((buf[(i) / ISEQ_MBITS_BITLENGTH] >> ((i) % ISEQ_MBITS_BITLENGTH)) & 0x1)
-#define ISEQ_MBITS_BUFLEN(size) (((size + (ISEQ_MBITS_BITLENGTH - 1)) & -ISEQ_MBITS_BITLENGTH) / ISEQ_MBITS_BITLENGTH)
+#define ISEQ_MBITS_BUFLEN(size) roomof(size, ISEQ_MBITS_BITLENGTH)
 
 #ifndef USE_ISEQ_NODE_ID
 #define USE_ISEQ_NODE_ID 1