diff --git a/src/borg/cache.py b/src/borg/cache.py
index d304f4408..b0ff5e541 100644
--- a/src/borg/cache.py
+++ b/src/borg/cache.py
@@ -1046,6 +1046,9 @@ class LocalCache(CacheStatsMixin):
         elif 'm' in cache_mode:
             cmtime_type = 'mtime'
             cmtime_ns = safe_ns(st.st_mtime_ns)
+        else:  # neither 'c' nor 'm' in cache_mode, avoid UnboundLocalError
+            cmtime_type = 'ctime'
+            cmtime_ns = safe_ns(st.st_ctime_ns)
         entry = FileCacheEntry(age=0, inode=st.st_ino, size=st.st_size, cmtime=int_to_bigint(cmtime_ns), chunk_ids=ids)
         self.files[path_hash] = msgpack.packb(entry)
         self._newest_cmtime = max(self._newest_cmtime or 0, cmtime_ns)