diff --git a/src/basic/io-util.h b/src/basic/io-util.h
index d98817f760..d4bb14fba4 100644
--- a/src/basic/io-util.h
+++ b/src/basic/io-util.h
@@ -33,14 +33,17 @@ static inline size_t IOVEC_TOTAL_SIZE(const struct iovec *i, size_t n) {
         return r;
 }
 
-static inline size_t IOVEC_INCREMENT(struct iovec *i, size_t n, size_t k) {
-        size_t j;
+static inline bool IOVEC_INCREMENT(struct iovec *i, size_t n, size_t k) {
+        /* Returns true if there is nothing else to send (bytes written cover all of the iovec),
+         * false if there's still work to do. */
 
-        for (j = 0; j < n; j++) {
+        for (size_t j = 0; j < n; j++) {
                 size_t sub;
 
-                if (_unlikely_(k <= 0))
-                        break;
+                if (i[j].iov_len == 0)
+                        continue;
+                if (k == 0)
+                        return false;
 
                 sub = MIN(i[j].iov_len, k);
                 i[j].iov_len -= sub;
@@ -48,7 +51,9 @@ static inline size_t IOVEC_INCREMENT(struct iovec *i, size_t n, size_t k) {
                 k -= sub;
         }
 
-        return k;
+        assert(k == 0); /* Anything else would mean that we wrote more bytes than available,
+                         * or the kernel reported writing more bytes than sent. */
+        return true;
 }
 
 static inline bool FILE_SIZE_VALID(uint64_t l) {
diff --git a/src/basic/log.c b/src/basic/log.c
index 595db0c395..c098939ed1 100644
--- a/src/basic/log.c
+++ b/src/basic/log.c
@@ -491,11 +491,11 @@ static int write_to_syslog(
                 if (n < 0)
                         return -errno;
 
-                if (!syslog_is_stream ||
-                    (size_t) n >= IOVEC_TOTAL_SIZE(iovec, ELEMENTSOF(iovec)))
+                if (!syslog_is_stream)
                         break;
 
-                IOVEC_INCREMENT(iovec, ELEMENTSOF(iovec), n);
+                if (IOVEC_INCREMENT(iovec, ELEMENTSOF(iovec), n))
+                        break;
         }
 
         return 1;