diff --git a/vivarium/core/serialize_test.py b/vivarium/core/serialize_test.py
index 911b72a3..fa8d5071 100644
--- a/vivarium/core/serialize_test.py
+++ b/vivarium/core/serialize_test.py
@@ -197,7 +197,7 @@ def test_non_string_keys() -> None:
     except TypeError as e:
         expected_error = (
             "These paths end in incompatible non-string or Numpy string " +
-            "keys: [('1',), (1,), ('string', 'string2', 'string3', '1')]")
+            "keys: [(np.str_('1'),), (1,), ('string', 'string2', 'string3', np.str_('1'))]")
         assert str(e) == expected_error
 
 
@@ -211,7 +211,7 @@ def test_unsupported_types() -> None:
     except TypeError as e:
         expected_error = (
             "These paths end in incompatible non-string or Numpy string " +
-            "keys: [('bad string',)]")
+            "keys: [(np.str_('bad string'),)]")
         assert str(e) == expected_error
         assert str(e.__cause__) == 'Type is not JSON serializable: type'