diff --git a/compat/mingw.h b/compat/mingw.h
index b4eaf85d41..820fe68de9 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -229,7 +229,7 @@ static inline ssize_t mingw_write(int fd, const void *buf, size_t len)
 {
 	ssize_t result = write(fd, buf, len);
 
-	if (result < 0 && errno == EINVAL) {
+	if (result < 0 && errno == EINVAL && buf) {
 		/* check if fd is a pipe */
 		HANDLE h = (HANDLE) _get_osfhandle(fd);
 		if (GetFileType(h) == FILE_TYPE_PIPE)