diff --git a/qutebrowser/completion/models/histcategory.py b/qutebrowser/completion/models/histcategory.py
index a07f78143..60f801492 100644
--- a/qutebrowser/completion/models/histcategory.py
+++ b/qutebrowser/completion/models/histcategory.py
@@ -80,7 +80,7 @@ class HistoryCategory(QSqlQueryModel):
             for i in range(len(words)))
 
         # replace ' in timestamp-format to avoid breaking the query
-        timestamp_format = config.val.completion.timestamp_format
+        timestamp_format = config.val.completion.timestamp_format or ''
         timefmt = ("strftime('{}', last_atime, 'unixepoch', 'localtime')"
                    .format(timestamp_format.replace("'", "`")))
 
diff --git a/tests/unit/completion/test_histcategory.py b/tests/unit/completion/test_histcategory.py
index d03098090..e7d7d8d28 100644
--- a/tests/unit/completion/test_histcategory.py
+++ b/tests/unit/completion/test_histcategory.py
@@ -205,3 +205,20 @@ def test_remove_rows_fetch(hist):
     hist.delete('url', '298')
     cat.removeRows(297, 1)
     assert cat.rowCount() == 299
+
+
+@pytest.mark.parametrize('fmt, expected', [
+    ('%Y-%m-%d', '2018-02-27'),
+    ('%m/%d/%Y %H:%M', '02/27/2018 08:30'),
+    ('', ''),
+])
+def test_timestamp_fmt(fmt, expected, model_validator, config_stub, init_sql):
+    """Validate the filtering and sorting results of set_pattern."""
+    config_stub.val.completion.timestamp_format = fmt
+    hist = sql.SqlTable('CompletionHistory', ['url', 'title', 'last_atime'])
+    atime = datetime.datetime(2018, 2, 27, 8, 30)
+    hist.insert({'url': 'foo', 'title': '', 'last_atime': atime.timestamp()})
+    cat = histcategory.HistoryCategory()
+    model_validator.set_model(cat)
+    cat.set_pattern('')
+    model_validator.validate([('foo', '', expected)])