diff --git a/src/basic/in-addr-util.c b/src/basic/in-addr-util.c
index 4bb7d9c5e0a..411efb242b9 100644
--- a/src/basic/in-addr-util.c
+++ b/src/basic/in-addr-util.c
@@ -362,7 +362,7 @@ int in_addr_ifindex_from_string_auto(const char *s, int *family, union in_addr_u
 unsigned char in4_addr_netmask_to_prefixlen(const struct in_addr *addr) {
         assert(addr);
 
-        return 32 - u32ctz(be32toh(addr->s_addr));
+        return 32U - u32ctz(be32toh(addr->s_addr));
 }
 
 struct in_addr* in4_addr_prefixlen_to_netmask(struct in_addr *addr, unsigned char prefixlen) {
diff --git a/src/basic/util.h b/src/basic/util.h
index 01118bb8954..85a7a15843c 100644
--- a/src/basic/util.h
+++ b/src/basic/util.h
@@ -193,7 +193,7 @@ static inline unsigned u64log2(uint64_t n) {
 
 static inline unsigned u32ctz(uint32_t n) {
 #if __SIZEOF_INT__ == 4
-        return __builtin_ctz(n);
+        return n != 0 ? __builtin_ctz(n) : 32;
 #else
 #error "Wut?"
 #endif