diff --git a/src/storage/storage_backend.c b/src/storage/storage_backend.c
index c29b162344..6915b8a10b 100644
--- a/src/storage/storage_backend.c
+++ b/src/storage/storage_backend.c
@@ -1444,6 +1444,10 @@ virStorageBackendDetectBlockVolFormatFD(virStorageSourcePtr target,
         }
     }
 
+    if (target->format == VIR_STORAGE_POOL_DISK_UNKNOWN)
+        VIR_DEBUG("cannot determine the target format for '%s'",
+                  target->path);
+
     return 0;
 }