diff --git a/src/labthings/apispec/plugins.py b/src/labthings/apispec/plugins.py
index 6a384d3..4339fdf 100644
--- a/src/labthings/apispec/plugins.py
+++ b/src/labthings/apispec/plugins.py
@@ -112,11 +112,10 @@ def spec_for_interaction(cls, interaction):
                 )
         return d
 
-    @classmethod
-    def spec_for_property(cls, prop):
-        class_schema = ensure_schema(prop.schema) or {}
+    def spec_for_property(self, prop):
+        class_schema = ensure_schema(self.spec, prop.schema) or {}
 
-        d = cls.spec_for_interaction(prop)
+        d = self.spec_for_interaction(prop)
 
         # Add in writeproperty methods
         for method in ("put", "post"):
@@ -155,9 +154,11 @@ def spec_for_property(cls, prop):
         return d
 
     def spec_for_action(self, action):
-        action_input = ensure_schema(action.args, name=f"{action.__name__}InputSchema")
+        action_input = ensure_schema(
+            self.spec, action.args, name=f"{action.__name__}InputSchema"
+        )
         action_output = ensure_schema(
-            action.schema, name=f"{action.__name__}OutputSchema"
+            self.spec, action.schema, name=f"{action.__name__}OutputSchema"
         )
         # We combine input/output parameters with ActionSchema using an
         # allOf directive, so we don't end up duplicating the schema
diff --git a/src/labthings/apispec/utilities.py b/src/labthings/apispec/utilities.py
index 6752119..7d42733 100644
--- a/src/labthings/apispec/utilities.py
+++ b/src/labthings/apispec/utilities.py
@@ -1,21 +1,27 @@
 from inspect import isclass
 from typing import Dict, Type, Union, cast
 
+from apispec import APISpec
 from apispec.ext.marshmallow import MarshmallowPlugin
-from apispec.ext.marshmallow.field_converter import FieldConverterMixin
 from marshmallow import Schema
 
 from .. import fields
 
 
-def field2property(field):
-    """Convert a marshmallow Field to OpenAPI dictionary"""
-    converter = FieldConverterMixin()
-    converter.init_attribute_functions()
-    return converter.field2property(field)
+def field2property(spec: APISpec, field: fields.Field):
+    """Convert a marshmallow Field to OpenAPI dictionary
+
+    We require an initialised APISpec object to use its
+    converter function - in particular, this will depend
+    on the OpenAPI version defined in `spec`.  We also rely
+    on the spec having a `MarshmallowPlugin` attached.
+    """
+    plugin = get_marshmallow_plugin(spec)
+    return plugin.converter.field2property(field)
 
 
 def ensure_schema(
+    spec: APISpec,
     schema: Union[
         fields.Field,
         Type[fields.Field],
@@ -34,11 +40,14 @@ def ensure_schema(
 
     Other Schemas are returned as Marshmallow Schema instances, which will be
     converted to references by the plugin.
+
+    The first argument must be an initialised APISpec object, as the conversion
+    of single fields to dictionaries is version-dependent.
     """
     if schema is None:
         return None
     if isinstance(schema, fields.Field):
-        return field2property(schema)
+        return field2property(spec, schema)
     elif isinstance(schema, dict):
         return Schema.from_dict(schema, name=name)()
     elif isinstance(schema, Schema):
@@ -46,7 +55,7 @@ def ensure_schema(
     if isclass(schema):
         schema = cast(Type, schema)
         if issubclass(schema, fields.Field):
-            return field2property(schema())
+            return field2property(spec, schema())
         elif issubclass(schema, Schema):
             return schema()
     raise TypeError(
diff --git a/tests/conftest.py b/tests/conftest.py
index 8a2a5e0..b698039 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -213,6 +213,17 @@ def post(self, args):
 
     thing.add_view(TestFieldProperty, "/TestFieldProperty")
 
+    class TestNullableFieldProperty(PropertyView):
+        schema = fields.Integer(allow_none=True)
+
+        def get(self):
+            return "one"
+
+        def post(self, args):
+            pass
+
+    thing.add_view(TestNullableFieldProperty, "/TestNullableFieldProperty")
+
     class FailAction(ActionView):
         wait_for = 0.1
 
diff --git a/tests/test_openapi.py b/tests/test_openapi.py
index 71786f5..5a81a5e 100644
--- a/tests/test_openapi.py
+++ b/tests/test_openapi.py
@@ -54,43 +54,49 @@ def post(self):
     assert original_input_schema != modified_input_schema
 
 
-def test_ensure_schema_field_instance():
-    ret = utilities.ensure_schema(fields.Integer())
+def test_ensure_schema_field_instance(spec):
+    ret = utilities.ensure_schema(spec, fields.Integer())
     assert ret == {"type": "integer"}
 
 
-def test_ensure_schema_field_class():
-    ret = utilities.ensure_schema(fields.Integer)
+def test_ensure_schema_nullable_field_instance(spec):
+    ret = utilities.ensure_schema(spec, fields.Integer(allow_none=True))
+    assert ret == {"type": "integer", "nullable": True}
+
+
+def test_ensure_schema_field_class(spec):
+    ret = utilities.ensure_schema(spec, fields.Integer)
     assert ret == {"type": "integer"}
 
 
-def test_ensure_schema_class():
-    ret = utilities.ensure_schema(LogRecordSchema)
+def test_ensure_schema_class(spec):
+    ret = utilities.ensure_schema(spec, LogRecordSchema)
     assert isinstance(ret, Schema)
 
 
-def test_ensure_schema_instance():
-    ret = utilities.ensure_schema(LogRecordSchema())
+def test_ensure_schema_instance(spec):
+    ret = utilities.ensure_schema(spec, LogRecordSchema())
     assert isinstance(ret, Schema)
 
 
-def test_ensure_schema_dict():
+def test_ensure_schema_dict(spec):
     ret = utilities.ensure_schema(
+        spec,
         {
             "count": fields.Integer(),
             "name": fields.String(),
-        }
+        },
     )
     assert isinstance(ret, Schema)
 
 
-def test_ensure_schema_none():
-    assert utilities.ensure_schema(None) is None
+def test_ensure_schema_none(spec):
+    assert utilities.ensure_schema(spec, None) is None
 
 
-def test_ensure_schema_error():
+def test_ensure_schema_error(spec):
     with pytest.raises(TypeError):
-        utilities.ensure_schema(Exception)
+        utilities.ensure_schema(spec, Exception)
 
 
 def test_get_marshmallow_plugin(spec):