diff --git a/dbignite/fhir_mapping_model.py b/dbignite/fhir_mapping_model.py
index 73bf676..d7cd37f 100644
--- a/dbignite/fhir_mapping_model.py
+++ b/dbignite/fhir_mapping_model.py
@@ -99,7 +99,7 @@ def us_core_fhir_resource_mapping(cls):
     #
     def custom_fhir_resource_mapping(self, resource_list: list[str]) -> "FhirSchemaModel":
         custom_mapping = {
-            resource_type: FhirSchemaModel.__read_schema(schema_path)
+            resource_type: FhirSchemaModel._read_schema(schema_path)
             for resource_type, schema_path in self._get_schema_paths()
             if resource_type in resource_list
         }
diff --git a/tests/test_writers.py b/tests/test_writers.py
index a89a365..c345eef 100644
--- a/tests/test_writers.py
+++ b/tests/test_writers.py
@@ -10,13 +10,13 @@ def test_encoder(self):
         from dbignite.writer.fhir_encoder import FhirEncoder
         e = FhirEncoder(False, False, lambda x: int(x.strip()))
         assert( e.f("123") == 123 )
-        assert( e.f("12a") is None ) 
+        assert( e.f("12a") == "" ) 
 
     def test_encoder_manager(self):
         from dbignite.writer.fhir_encoder import FhirEncoderManager, SchemaDataType, FhirEncoder
         em = FhirEncoderManager()
         assert(em.get_encoder("string", ["Patient", "multipleBirthInteger"]).f("1234") == 1234)
-        assert(em.get_encoder("string", ["Patient", "multipleBirthInteger"]).f("abcdefg") == None)
+        assert(em.get_encoder("string", ["Patient", "multipleBirthInteger"]).f("abcdefg") == "")
 
         #test overrides
         em = FhirEncoderManager(override_encoders = {"Patient.multipleBirthInteger": FhirEncoder(False, False, lambda x: float(x) + 1)})
@@ -27,7 +27,8 @@ def test_target_datatype(self):
         from dbignite.writer.fhir_encoder import SchemaDataType
         from dbignite.fhir_mapping_model import FhirSchemaModel
         field = "Patient.name.given"
-        tgt_schema = FhirSchemaModel.custom_fhir_resource_mapping(field.split(".")[0]).schema(field.split(".")[0])
+        s = FhirSchemaModel()
+        tgt_schema = s.custom_fhir_resource_mapping(field.split(".")[0]).schema(field.split(".")[0])
         result = SchemaDataType.traverse_schema(field.split(".")[1:], tgt_schema)
         assert( result == "array<string>")