diff --git a/test/test_compound_identifier.py b/test/test_compound_identifier.py
index e0cc5bd7b..d89893330 100644
--- a/test/test_compound_identifier.py
+++ b/test/test_compound_identifier.py
@@ -9,6 +9,10 @@ def compound_identifier():
 def test_is_input_header_positive(compound_identifier, header):
     """Test that valid input headers return True."""
     assert compound_identifier.is_input_header(header) is True
+@pytest.mark.parametrize("header", ["output", "invalid", "InChI", "inchikey"])
+def test_is_input_header_negative(compound_identifier, header):
+    """Test that invalid input headers return False."""
+    assert compound_identifier.is_input_header(header) is False
     
 @pytest.mark.parametrize("header", ["key", "inchiKey", "KEY", "INCHIKEY"])
 def test_is_key_header_positive(compound_identifier, header):