diff --git a/smartnoise_synth_logger/deserialise.py b/smartnoise_synth_logger/deserialise.py
index 4dec0f6..10f81ae 100644
--- a/smartnoise_synth_logger/deserialise.py
+++ b/smartnoise_synth_logger/deserialise.py
@@ -82,7 +82,9 @@ def deserialise_constraints(constraints_json: str) -> dict:
 
     deserialised = {}
     for key, val in json_body[JsonBodyKey.CONSTRAINTS].items():
-        if isinstance(val[JsonBodyKey.PARAM], list):
+        if isinstance(val, str):
+            deserialised[key] = val
+        elif isinstance(val[JsonBodyKey.PARAM], list):
             tranformer_list = []
             for t in val[JsonBodyKey.PARAM]:
                 tranformer_list.append(
diff --git a/smartnoise_synth_logger/serialise.py b/smartnoise_synth_logger/serialise.py
index 4c687af..0b0048a 100644
--- a/smartnoise_synth_logger/serialise.py
+++ b/smartnoise_synth_logger/serialise.py
@@ -54,23 +54,28 @@ def serialise_constraints(constraints: dict) -> str:
     }
 
     for col_name, col_constraints in constraints.items():
-        operator_name = col_constraints.__class__.__name__
+        if isinstance(col_constraints, str):
+            json_body[JsonBodyKey.CONSTRAINTS][col_name] = col_constraints
+        else:
+            operator_name = col_constraints.__class__.__name__
 
-        if operator_name == Transformers.CHAIN:
-            transformer_dict = handle_chain_transformer(col_constraints)
-        elif operator_name == Transformers.ANONIMIZATION:
-            transformer_dict = {
-                JsonBodyKey.TYPE: SSYNTH_TRANSFORMER
-                + Transformers.ANONIMIZATION,
-                JsonBodyKey.PARAM: {ANON_PARAM: col_constraints.fake.__name__},
-            }
-        else:  # default
-            transformer_dict = {
-                JsonBodyKey.TYPE: SSYNTH_TRANSFORMER
-                + col_constraints.__class__.__name__,
-                JsonBodyKey.PARAM: get_filtered_params(col_constraints),
-            }
+            if operator_name == Transformers.CHAIN:
+                transformer_dict = handle_chain_transformer(col_constraints)
+            elif operator_name == Transformers.ANONIMIZATION:
+                transformer_dict = {
+                    JsonBodyKey.TYPE: SSYNTH_TRANSFORMER
+                    + Transformers.ANONIMIZATION,
+                    JsonBodyKey.PARAM: {
+                        ANON_PARAM: col_constraints.fake.__name__
+                    },
+                }
+            else:  # default
+                transformer_dict = {
+                    JsonBodyKey.TYPE: SSYNTH_TRANSFORMER
+                    + col_constraints.__class__.__name__,
+                    JsonBodyKey.PARAM: get_filtered_params(col_constraints),
+                }
 
-        json_body[JsonBodyKey.CONSTRAINTS][col_name] = transformer_dict
+            json_body[JsonBodyKey.CONSTRAINTS][col_name] = transformer_dict
 
     return json.dumps(json_body)
diff --git a/tests/test_de_serialisation.py b/tests/test_de_serialisation.py
index eee5cc8..d2d2b29 100644
--- a/tests/test_de_serialisation.py
+++ b/tests/test_de_serialisation.py
@@ -29,6 +29,17 @@ def test_anon_serialize():
     assert result_json == expected_json_updated
 
 
+def test_anon_str_serialize():
+    example_constraints = {"id": "email"}
+    result_json = serialise_constraints(example_constraints)
+
+    expected_json = """{"module": "smartnoise-synth", "version": "1.0.4", "constraints": {"id": "email"}}"""  # noqa
+    expected_json_updated = expected_json.replace(
+        "1.0.4", pkg_resources.get_distribution(SSYNTH).version
+    )
+    assert result_json == expected_json_updated
+
+
 def test_chain_serialize():
     example_constraints = {
         "income": ChainTransformer(