Test invalid arguments to area constraint

tests/operators/test_constraints.py

@@ -16,6 +16,8 @@
 
 from datetime import datetime
 
+import pytest
+
 from CSET.operators import constraints
 
 
@@ -133,6 +135,17 @@ def test_generate_area_constraint_no_limits():
     assert expected_area_constraint in repr(area_constraint)
 
 
+def test_generate_area_constraint_invalid_arguments():
+    """Generate area constraint raises exception with invalid arguments."""
+    # Non-numbers are rejected.
+    with pytest.raises(TypeError):
+        constraints.generate_area_constraint(1, 2, 3, "four")
+
+    # Mixed numbers and Nones are rejected.
+    with pytest.raises(TypeError):
+        constraints.generate_area_constraint(None, None, None, 0)
+
+
 def test_combine_constraints():
     """Combine constraint."""
     stash_constraint = constraints.generate_stash_constraint("m01s03i236")