diff --git a/augur/filter.py b/augur/filter.py
index c850f3ed8..c29608cdf 100644
--- a/augur/filter.py
+++ b/augur/filter.py
@@ -1734,6 +1734,10 @@ def numeric_date(date):
     except (ValueError, isodate.ISO8601Error):
         pass
 
+    # This currently doesn't get exposed since argparse raises a SystemExit on invalid arguments.
+    # TODO: find a way to provide better errors for invalid dates.
+    raise ValueError(f"Unable to determine date from {date}.")
+
 
 def calculate_sequences_per_group(target_max_value, counts_per_group, allow_probabilistic=True):
     """Calculate the number of sequences per group for a given maximum number of
diff --git a/tests/test_filter.py b/tests/test_filter.py
index 248c1706d..f2f087579 100644
--- a/tests/test_filter.py
+++ b/tests/test_filter.py
@@ -356,3 +356,19 @@ def test_filter_relative_dates(self, tmpdir, argparser, argparse_params, metadat
         with open(out_fn) as f:
             output_sorted = sorted(line.rstrip() for line in f)
         assert output_sorted == output_sorted_expected
+
+    @freeze_time("2020-03-25")
+    @pytest.mark.parametrize(
+        "argparse_params",
+        [
+            "--max-date 3000Y",
+            "--max-date invalid",
+        ],
+    )
+    def test_filter_relative_dates_error(self, tmpdir, argparser, argparse_params):
+        """Test that invalid dates fail"""
+        out_fn = str(tmpdir / "filtered.txt")
+        meta_fn = write_metadata(tmpdir, (("strain","date"),
+                                          ("SEQ_1","2020-03-23")))
+        with pytest.raises(SystemExit):
+            argparser(f'--metadata {meta_fn} --output-strains {out_fn} {argparse_params}')