diff --git a/src/fileops.c b/src/fileops.c
index ad2a988a9..f9552a5f8 100644
--- a/src/fileops.c
+++ b/src/fileops.c
@@ -304,7 +304,9 @@ int git_futils_mmap_ro_file(git_map *out, const char *path)
 	if (fd < 0)
 		return fd;
 
-	len = git_futils_filesize(fd);
+	if ((len = git_futils_filesize(fd)) < 0)
+		return -1;
+
 	if (!git__is_sizet(len)) {
 		giterr_set(GITERR_OS, "file `%s` too large to mmap", path);
 		return -1;