diff --git a/lfs.c b/lfs.c
index c94fa68e..2becf184 100644
--- a/lfs.c
+++ b/lfs.c
@@ -1291,6 +1291,7 @@ static lfs_stag_t lfs_dir_fetchmatch(lfs_t *lfs,
 
             // found a match for our fetcher?
             if ((fmask & tag) == (fmask & ftag)) {
+                LFS_ASSERT(cb != NULL);
                 int res = cb(data, tag, &(struct lfs_diskoff){
                         dir->pair[0], off+sizeof(tag)});
                 if (res < 0) {