diff --git a/gallery_dl/postprocessor/metadata.py b/gallery_dl/postprocessor/metadata.py
index 93dd9a19..18d00e13 100644
--- a/gallery_dl/postprocessor/metadata.py
+++ b/gallery_dl/postprocessor/metadata.py
@@ -189,7 +189,7 @@ class MetadataPP(PostProcessor):
             tags = []
             extend = tags.extend
             for tagdict in taglists:
-                extend([x for x in tagdict.values() if x is not None])
+                extend([x for x in tagdict.values() if isinstance(x, str)])
             tags.sort()
 
         fp.write("\n".join(tags) + "\n")
diff --git a/test/test_postprocessor.py b/test/test_postprocessor.py
index b64df882..fb1d739e 100644
--- a/test/test_postprocessor.py
+++ b/test/test_postprocessor.py
@@ -365,8 +365,8 @@ class MetadataTest(BasePostprocessorTest):
         self._create(
             {"mode": "tags"},
             {"tags": [
-                {"g": "foobar1", "m": "foobar2"},
-                {"g": None, "m": "foobarbaz"}
+                {"g": "foobar1", "m": "foobar2", "u": True},
+                {"g": None, "m": "foobarbaz", "u": [3, 4]},
             ]},
         )
         with patch("builtins.open", mock_open()) as m: