diff --git a/src/apispec/ext/marshmallow/field_converter.py b/src/apispec/ext/marshmallow/field_converter.py
index a284f9b1..63fbe967 100644
--- a/src/apispec/ext/marshmallow/field_converter.py
+++ b/src/apispec/ext/marshmallow/field_converter.py
@@ -301,9 +301,13 @@ def field2nullable(self, field: marshmallow.fields.Field, ret) -> dict:
             if self.openapi_version.major < 3:
                 attributes["x-nullable"] = True
             elif self.openapi_version.minor < 1:
-                attributes["nullable"] = True
                 if "$ref" in ret:
-                    attributes["allOf"] = [{"$ref": ret.pop("$ref")}]
+                    attributes["anyOf"] = [
+                        {"type": "object", "nullable": True},
+                        {"$ref": ret.pop("$ref")},
+                    ]
+                else:
+                    attributes["nullable"] = True
             else:
                 if "$ref" in ret:
                     attributes["anyOf"] = [{"$ref": ret.pop("$ref")}, {"type": "null"}]
@@ -580,9 +584,11 @@ def datetime2properties(self, field, **kwargs: typing.Any) -> dict:
                 ret = {
                     "type": "string",
                     "format": None,
-                    "pattern": field.metadata["pattern"]
-                    if field.metadata.get("pattern")
-                    else None,
+                    "pattern": (
+                        field.metadata["pattern"]
+                        if field.metadata.get("pattern")
+                        else None
+                    ),
                 }
         return ret
 
diff --git a/tests/test_ext_marshmallow_field.py b/tests/test_ext_marshmallow_field.py
index 20e079d2..5e5250b0 100644
--- a/tests/test_ext_marshmallow_field.py
+++ b/tests/test_ext_marshmallow_field.py
@@ -211,8 +211,10 @@ class Child(Schema):
         assert res == {"$ref": "#/definitions/Child", "x-nullable": True}
     elif version.major == 3 and version.minor < 1:
         assert res == {
-            "nullable": True,
-            "allOf": [{"$ref": "#/components/schemas/Child"}],
+            "anyOf": [
+                {"type": "object", "nullable": True},
+                {"$ref": "#/components/schemas/Child"},
+            ]
         }
     else:
         assert res == {