diff --git a/src/sourmash/manifest.py b/src/sourmash/manifest.py
index ad8a72abf..549f53c82 100644
--- a/src/sourmash/manifest.py
+++ b/src/sourmash/manifest.py
@@ -125,6 +125,8 @@ def write_to_filename(self, filename, *, database_format="csv", ok_if_exists=Fal
             SqliteCollectionManifest.load_from_manifest(
                 self, dbfile=filename, append=ok_if_exists
             )
+        else:
+            raise Exception("unknown output format for manifest")
 
     @classmethod
     def write_csv_header(cls, fp):