diff --git a/graphql_schema/lib/src/union.dart b/graphql_schema/lib/src/union.dart
index ed7f9a4e..2d7b0360 100644
--- a/graphql_schema/lib/src/union.dart
+++ b/graphql_schema/lib/src/union.dart
@@ -1,22 +1,25 @@
 part of graphql_schema.src.schema;
 
-class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
-    with _NonNullableMixin<Value, Serialized> {
-  final List<GraphQLType<Value, Serialized>> possibleTypes;
-  final String description;
+class GraphQLUnionType
+    extends GraphQLType<Map<String, dynamic>, Map<String, dynamic>>
+    with _NonNullableMixin<Map<String, dynamic>, Map<String, dynamic>> {
+  final String name;
+  final List<GraphQLObjectType> possibleTypes = [];
 
-  GraphQLUnionType(this.possibleTypes, {this.description}) {
-    assert(possibleTypes.every((t) => t is GraphQLUnionType),
-        'The member types of a Union type must all be Object base types; Scalar, Interface and Union types may not be member types of a Union. Similarly, wrapping types may not be member types of a Union');
+  GraphQLUnionType(
+    this.name,
+    Iterable<GraphQLObjectType> possibleTypes,
+  ) {
     assert(possibleTypes.isNotEmpty,
-        'A Union type must define one or more member types');
+        'A Union type must define one or more member types.');
+    this.possibleTypes.addAll(possibleTypes.toSet());
   }
 
   @override
-  String get name => possibleTypes.map((t) => t.name).join(' | ');
+  String get description => possibleTypes.map((t) => t.name).join(' | ');
 
   @override
-  Serialized serialize(Value value) {
+  Map<String, dynamic> serialize(Map<String, dynamic> value) {
     for (var type in possibleTypes) {
       try {
         return type.serialize(value);
@@ -27,7 +30,7 @@ class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
   }
 
   @override
-  Value deserialize(Serialized serialized) {
+  Map<String, dynamic> deserialize(Map<String, dynamic> serialized) {
     for (var type in possibleTypes) {
       try {
         return type.deserialize(serialized);
@@ -38,7 +41,8 @@ class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
   }
 
   @override
-  ValidationResult<Serialized> validate(String key, Serialized input) {
+  ValidationResult<Map<String, dynamic>> validate(
+      String key, Map<String, dynamic> input) {
     List<String> errors = [];
 
     for (var type in possibleTypes) {
@@ -51,6 +55,6 @@ class GraphQLUnionType<Value, Serialized> extends GraphQLType<Value, Serialized>
       }
     }
 
-    return new ValidationResult._failure(errors);
+    return new ValidationResult<Map<String, dynamic>>._failure(errors);
   }
 }
diff --git a/graphql_server/lib/graphql_server.dart b/graphql_server/lib/graphql_server.dart
index 01cab90a..8410a12a 100644
--- a/graphql_server/lib/graphql_server.dart
+++ b/graphql_server/lib/graphql_server.dart
@@ -6,22 +6,31 @@ import 'package:graphql_schema/graphql_schema.dart';
 import 'introspection.dart';
 
 class GraphQL {
-  final Map<String, GraphQLType> customTypes = {};
+  final List<GraphQLType> customTypes = [];
   GraphQLSchema _schema;
 
-  GraphQL(GraphQLSchema schema, {bool introspect: true}) : _schema = schema {
+  GraphQL(GraphQLSchema schema,
+      {bool introspect: true,
+      List<GraphQLType> customTypes = const <GraphQLType>[]})
+      : _schema = schema {
+    if (customTypes?.isNotEmpty == true) {
+      this.customTypes.addAll(customTypes);
+    }
+
     if (introspect) {
       var allTypes = <GraphQLType>[];
+      allTypes.addAll(this.customTypes);
       _schema = reflectSchema(_schema, allTypes);
 
       for (var type in allTypes) {
-        customTypes[type.name] = type;
+        if (!this.customTypes.contains(type)) {
+          this.customTypes.add(type);
+        }
       }
     }
 
-    if (_schema.query != null) customTypes[_schema.query.name] = _schema.query;
-    if (_schema.mutation != null)
-      customTypes[_schema.mutation.name] = _schema.mutation;
+    if (_schema.query != null) this.customTypes.add(_schema.query);
+    if (_schema.mutation != null) this.customTypes.add(_schema.mutation);
   }
 
   GraphQLType convertType(TypeContext ctx) {
@@ -43,11 +52,9 @@ class GraphQL {
         case 'DateTime':
           return graphQLDate;
         default:
-          if (customTypes.containsKey(ctx.typeName.name))
-            return customTypes[ctx.typeName.name];
-          throw new ArgumentError(
-              'Unknown GraphQL type: "${ctx.typeName.name}"');
-          break;
+          return customTypes.firstWhere((t) => t.name == ctx.typeName.name,
+              orElse: () => throw new ArgumentError(
+                  'Unknown GraphQL type: "${ctx.typeName.name}"'));
       }
     } else {
       throw new ArgumentError('Invalid GraphQL type: "${ctx.span.text}"');
@@ -435,7 +442,8 @@ class GraphQL {
 
     var vname = vv.variable.name;
     if (!variableValues.containsKey(vname))
-      throw new GraphQLException.fromSourceSpan('Unknown variable: "$vname"', vv.span);
+      throw new GraphQLException.fromSourceSpan(
+          'Unknown variable: "$vname"', vv.span);
 
     return variableValues[vname];
   }
diff --git a/graphql_server/lib/introspection.dart b/graphql_server/lib/introspection.dart
index 4f3148fb..921d2707 100644
--- a/graphql_server/lib/introspection.dart
+++ b/graphql_server/lib/introspection.dart
@@ -3,6 +3,16 @@ import 'package:graphql_schema/graphql_schema.dart';
 
 // TODO: How to handle custom types???
 GraphQLSchema reflectSchema(GraphQLSchema schema, List<GraphQLType> allTypes) {
+  for (var type in allTypes.toList()) {
+    var custom = _fetchAllTypesFromType(type);
+
+    for (var t in custom) {
+      if (!allTypes.contains(t)) {
+        allTypes.add(t);
+      }
+    }
+  }
+
   var objectTypes = fetchAllTypes(schema, allTypes);
   var typeType = _reflectSchemaTypes();
   var directiveType = _reflectDirectiveType();
@@ -210,8 +220,7 @@ GraphQLObjectType _createTypeType() {
         else if (t is GraphQLUnionType)
           return 'UNION';
         else
-          throw new UnsupportedError(
-              'Cannot get the kind of $t.');
+          throw new UnsupportedError('Cannot get the kind of $t.');
       },
     ),
     field(
@@ -395,25 +404,28 @@ GraphQLObjectType _reflectEnumValueType() {
       );
 }
 
-List<GraphQLObjectType> fetchAllTypes(
+List<GraphQLType> fetchAllTypes(
     GraphQLSchema schema, List<GraphQLType> allTypes) {
-  var typess = <GraphQLType>[];
-  typess.addAll(_fetchAllTypesFromObject(schema.query));
+  var types = <GraphQLType>[];
+
+  types.addAll(_fetchAllTypesFromObject(schema.query));
 
   if (schema.mutation != null) {
-    typess.addAll(_fetchAllTypesFromObject(schema.mutation)
+    types.addAll(_fetchAllTypesFromObject(schema.mutation)
         .where((t) => t is GraphQLObjectType));
   }
 
-  var types = <GraphQLObjectType>[];
+  return types;
 
-  for (var type in typess) {
-    if (type is GraphQLObjectType)
-      types.add(type);
-    else if (!allTypes.contains(type)) allTypes.add(type);
-  }
-
-  return types.toSet().toList();
+//  var types = <GraphQLObjectType>[];
+//
+//  for (var type in typess) {
+//    if (type is GraphQLObjectType)
+//      types.add(type);
+//    else if (!allTypes.contains(type)) allTypes.add(type);
+//  }
+//
+//  return types.toSet().toList();
 }
 
 List<GraphQLType> _fetchAllTypesFromObject(GraphQLObjectType objectType) {
@@ -449,7 +461,7 @@ Iterable<GraphQLType> _fetchAllTypesFromType(GraphQLType type) {
     types.addAll(_fetchAllTypesFromObject(type));
   } else if (type is GraphQLEnumType) {
     types.add(type);
-  } else if ( type is GraphQLUnionType) {
+  } else if (type is GraphQLUnionType) {
     for (var t in type.possibleTypes) {
       types.addAll(_fetchAllTypesFromType(t));
     }