diff --git a/include/asterisk/select.h b/include/asterisk/select.h
index a8f7c3c60..d6d34fbbd 100644
--- a/include/asterisk/select.h
+++ b/include/asterisk/select.h
@@ -43,12 +43,14 @@ typedef struct {
 	TYPEOF_FD_SET_FDS_BITS fds_bits[ast_FDMAX / 8 / SIZEOF_FD_SET_FDS_BITS]; /* 32768 bits */
 } ast_fdset;
 
+#define _bitsize(a)	(sizeof(a) * 8)
+
 #undef FD_ZERO
 #define FD_ZERO(a) \
 	do { \
 		TYPEOF_FD_SET_FDS_BITS *bytes = (TYPEOF_FD_SET_FDS_BITS *) a; \
 		int i; \
-		for (i = 0; i < sizeof(*(a)) / SIZEOF_FD_SET_FDS_BITS; i++) { \
+		for (i = 0; i < ast_FDMAX / _bitsize(TYPEOF_FD_SET_FDS_BITS); i++) { \
 			bytes[i] = 0; \
 		} \
 	} while (0)
@@ -56,8 +58,10 @@ typedef struct {
 #define FD_SET(fd, fds) \
 	do { \
 		TYPEOF_FD_SET_FDS_BITS *bytes = (TYPEOF_FD_SET_FDS_BITS *) fds; \
-		if (fd / sizeof(*bytes) + ((fd + 1) % sizeof(*bytes) ? 1 : 0) < sizeof(*(fds))) { \
-			bytes[fd / (sizeof(*bytes) * 8)] |= ((TYPEOF_FD_SET_FDS_BITS) 1) << (fd % (sizeof(*bytes) * 8)); \
+		/* 32bit: FD / 32 + ((FD + 1) % 32 ? 1 : 0) < 1024 */ \
+		/* 64bit: FD / 64 + ((FD + 1) % 64 ? 1 : 0) < 512 */ \
+		if (fd / _bitsize(*bytes) + ((fd + 1) % _bitsize(*bytes) ? 1 : 0) < sizeof(*(fds)) / SIZEOF_FD_SET_FDS_BITS) { \
+			bytes[fd / _bitsize(*bytes)] |= ((TYPEOF_FD_SET_FDS_BITS) 1) << (fd % _bitsize(*bytes)); \
 		} else { \
 			fprintf(stderr, "FD %d exceeds the maximum size of ast_fdset!\n", fd); \
 		} \