diff --git a/internal.h b/internal.h
index 7f9aaf6707..ff8bb6364e 100644
--- a/internal.h
+++ b/internal.h
@@ -625,8 +625,10 @@ struct RBignum {
 #define BIGNUM_NEGATE(b) (RBASIC(b)->flags ^= BIGNUM_SIGN_BIT)
 
 #define BIGNUM_EMBED_FLAG ((VALUE)FL_USER2)
-#define BIGNUM_EMBED_LEN_MASK ((VALUE)(FL_USER5|FL_USER4|FL_USER3))
-#define BIGNUM_EMBED_LEN_SHIFT (FL_USHIFT+BIGNUM_EMBED_LEN_NUMBITS)
+#define BIGNUM_EMBED_LEN_MASK \
+    (~(~(VALUE)0U << BIGNUM_EMBED_LEN_NUMBITS) << BIGNUM_EMBED_LEN_SHIFT)
+#define BIGNUM_EMBED_LEN_SHIFT \
+    (FL_USHIFT+3) /* bit offset of BIGNUM_EMBED_LEN_MASK */
 #define BIGNUM_LEN(b) \
     ((RBASIC(b)->flags & BIGNUM_EMBED_FLAG) ? \
      (size_t)((RBASIC(b)->flags >> BIGNUM_EMBED_LEN_SHIFT) & \