diff --git a/personal_data/main.py b/personal_data/main.py
index 50b940a..6cce2fe 100644
--- a/personal_data/main.py
+++ b/personal_data/main.py
@@ -99,6 +99,12 @@ def deduplicate_dicts(
     return dicts, fieldnames
 
 
+def normalize_dict(d: dict) -> frozendict:
+    return frozendict(
+        {k: to_value(str(v)) for k, v in d.items() if to_value(str(v)) is not None},
+    )
+
+
 def extend_csv_file(
     filename: str,
     new_dicts: list[dict],
@@ -123,7 +129,7 @@ def extend_csv_file(
         logger.info('Creating file: %s', filename)
 
     original_num_dicts = len(dicts)
-    dicts += [frozendict(d) for d in new_dicts]
+    dicts += [normalize_dict(d) for d in new_dicts]
     del new_dicts
 
     dicts, fieldnames = deduplicate_dicts(
diff --git a/test/test_deduplicate.py b/test/test_deduplicate.py
new file mode 100644
index 0000000..b484aa5
--- /dev/null
+++ b/test/test_deduplicate.py
@@ -0,0 +1,29 @@
+from frozendict import frozendict
+
+from personal_data.data import DeduplicateMode
+from personal_data.main import deduplicate_dicts
+
+LIST = [
+    frozendict({'a': 1, 'b': 2, 't': 300}),
+    frozendict({'a': 1, 'b': 2, 't': 301}),
+    frozendict({'a': 1, 'b': 2, 't': 302}),
+    frozendict({'a': 1, 'b': 2, 't': 303}),
+]
+
+
+def test_no_deduplicate():
+    ls, fields = deduplicate_dicts(LIST, DeduplicateMode.NONE, [])
+    assert fields == ['a', 'b', 't']
+    assert ls == LIST
+
+
+def test_only_latest_no_fields():
+    ls, fields = deduplicate_dicts(LIST, DeduplicateMode.ONLY_LATEST, [])
+    assert fields == ['a', 'b', 't']
+    assert ls == LIST
+
+
+def test_only_latest():
+    ls, fields = deduplicate_dicts(LIST, DeduplicateMode.ONLY_LATEST, ['t'])
+    assert fields == ['a', 'b', 't']
+    assert ls == [frozendict({'a': 1, 'b': 2, 't': 300})]