diff --git a/gql/dsl.py b/gql/dsl.py
index 1876742e..adc48bea 100644
--- a/gql/dsl.py
+++ b/gql/dsl.py
@@ -295,7 +295,11 @@ def __getattr__(self, name: str) -> "DSLType":
         if type_def is None:
             raise AttributeError(f"Type '{name}' not found in the schema!")
 
-        assert isinstance(type_def, (GraphQLObjectType, GraphQLInterfaceType))
+        if not isinstance(type_def, (GraphQLObjectType, GraphQLInterfaceType)):
+            raise AttributeError(
+                f'Type "{name} ({type_def!r})" is not valid as an attribute of'
+                " DSLSchema. Only Object types or Interface types are accepted."
+            )
 
         return DSLType(type_def, self)
 
diff --git a/tests/starwars/test_dsl.py b/tests/starwars/test_dsl.py
index 8bbdf0c9..098a2b50 100644
--- a/tests/starwars/test_dsl.py
+++ b/tests/starwars/test_dsl.py
@@ -14,6 +14,8 @@
     NonNullTypeNode,
     NullValueNode,
     Undefined,
+    build_ast_schema,
+    parse,
     print_ast,
 )
 from graphql.utilities import get_introspection_query
@@ -774,8 +776,6 @@ def test_dsl_query_all_fields_should_correspond_to_the_root_type(ds):
 
 def test_dsl_root_type_not_default():
 
-    from graphql import parse, build_ast_schema
-
     schema_str = """
 schema {
   query: QueryNotDefault
@@ -827,6 +827,41 @@ def test_invalid_type(ds):
         ds.invalid_type
 
 
+def test_invalid_type_union():
+    schema_str = """
+    type FloatValue {
+        floatValue: Float!
+    }
+
+    type IntValue {
+        intValue: Int!
+    }
+
+    union Value = FloatValue | IntValue
+
+    type Entry {
+        name: String!
+        value: Value
+    }
+
+    type Query {
+        values: [Entry!]!
+    }
+    """
+
+    schema = build_ast_schema(parse(schema_str))
+    ds = DSLSchema(schema)
+
+    with pytest.raises(
+        AttributeError,
+        match=(
+            "Type \"Value \\(<GraphQLUnionType 'Value'>\\)\" is not valid as an "
+            "attribute of DSLSchema. Only Object types or Interface types are accepted."
+        ),
+    ):
+        ds.Value
+
+
 def test_hero_name_query_with_typename(ds):
     query = """
 hero {