diff --git a/src/apispec/ext/marshmallow/field_converter.py b/src/apispec/ext/marshmallow/field_converter.py
index 3b837b44..9016e5f5 100644
--- a/src/apispec/ext/marshmallow/field_converter.py
+++ b/src/apispec/ext/marshmallow/field_converter.py
@@ -318,7 +318,7 @@ def field2nullable(self, field: marshmallow.fields.Field, ret) -> dict:
                     attributes["anyOf"] = [{"$ref": ret.pop("$ref")}, {"type": "null"}]
                 elif "allOf" in ret:
                     attributes["anyOf"] = [*ret.pop("allOf"), {"type": "null"}]
-                else:
+                elif "type" in ret:
                     attributes["type"] = [*make_type_list(ret.get("type")), "null"]
         return attributes
 
diff --git a/tests/test_ext_marshmallow_field.py b/tests/test_ext_marshmallow_field.py
index 36d864c4..a24f0a3f 100644
--- a/tests/test_ext_marshmallow_field.py
+++ b/tests/test_ext_marshmallow_field.py
@@ -199,6 +199,20 @@ def test_field_with_allow_none(spec_fixture):
         assert res["type"] == ["string", "null"]
 
 
+@pytest.mark.parametrize("spec_fixture", ("2.0", "3.0.0", "3.1.0"), indirect=True)
+@pytest.mark.parametrize("field_class", [fields.Field, fields.Raw])
+def test_nullable_field_with_no_type(spec_fixture, field_class):
+    field = field_class(allow_none=True)
+    res = spec_fixture.openapi.field2property(field)
+    if spec_fixture.openapi.openapi_version.major < 3:
+        assert res["x-nullable"] is True
+    elif spec_fixture.openapi.openapi_version.minor < 1:
+        assert res["nullable"] is True
+    else:
+        assert "nullable" not in res
+        assert "type" not in res
+
+
 @pytest.mark.parametrize("spec_fixture", ("2.0", "3.0.0", "3.1.0"), indirect=True)
 def test_nested_nullable(spec_fixture):
     class Child(Schema):