diff --git a/lib/graphql/autotest/query_generator.rb b/lib/graphql/autotest/query_generator.rb
index 8d705b1..5b6411b 100644
--- a/lib/graphql/autotest/query_generator.rb
+++ b/lib/graphql/autotest/query_generator.rb
@@ -70,8 +70,20 @@ def generate
       end
 
       private def type_definition(name)
-        document.definitions.find { |f| f.name == name }
+        defs = document.definitions.select { |f| f.name == name }
+        return defs.first if defs.size <= 1
+
+        MergedDefinition.new(name, defs.flat_map(&:fields))
+      end
+    end
+
+    class MergedDefinition
+      attr_reader :name, :fields
+      def initialize(name, fields)
+        @name = name
+        @fields = fields
       end
     end
+    private_constant :MergedDefinition
   end
 end
diff --git a/test/graphql/autotest/query_generator_test.rb b/test/graphql/autotest/query_generator_test.rb
index a5af625..a62a213 100644
--- a/test/graphql/autotest/query_generator_test.rb
+++ b/test/graphql/autotest/query_generator_test.rb
@@ -353,6 +353,30 @@ def test_sub_field_argument_schema
     GRAPHQL
   end
 
+  def test_extended_type_schema
+    schema = <<~GRAPHQL
+      type Item {
+        title: String!
+      }
+      extend type Item {
+        price: Int!
+      }
+      type Query {
+        item: Item!
+      }
+    GRAPHQL
+    document = GraphQL::parse(schema)
+    fields = GraphQL::Autotest::QueryGenerator.generate(document: document)
+
+    assert_query [<<~GRAPHQL, '__typename'], fields
+      item {
+        price
+        title
+        __typename
+      }
+    GRAPHQL
+  end
+
   private def generate(schema:, **kw)
     GraphQL::Autotest::QueryGenerator.generate(document: schema.to_document, **kw)
   end