diff --git a/validator/schema.go b/validator/schema.go
index a979bc41..d49ff93b 100644
--- a/validator/schema.go
+++ b/validator/schema.go
@@ -34,6 +34,8 @@ func ValidateSchemaDocument(ast *SchemaDocument) (*Schema, *gqlerror.Error) {
 		schema.Types[def.Name] = ast.Definitions[i]
 	}
 
+	defs := append(DefinitionList{}, ast.Definitions...)
+
 	for _, ext := range ast.Extensions {
 		def := schema.Types[ext.Name]
 		if def == nil {
@@ -43,6 +45,7 @@ func ValidateSchemaDocument(ast *SchemaDocument) (*Schema, *gqlerror.Error) {
 				Position: ext.Position,
 			}
 			def = schema.Types[ext.Name]
+			defs = append(defs, def)
 		}
 
 		if def.Kind != ext.Kind {
@@ -56,7 +59,7 @@ func ValidateSchemaDocument(ast *SchemaDocument) (*Schema, *gqlerror.Error) {
 		def.EnumValues = append(def.EnumValues, ext.EnumValues...)
 	}
 
-	for _, def := range ast.Definitions {
+	for _, def := range defs {
 		switch def.Kind {
 		case Union:
 			for _, t := range def.Types {
diff --git a/validator/schema_test.yml b/validator/schema_test.yml
index 5bbf2ada..a5d8bb49 100644
--- a/validator/schema_test.yml
+++ b/validator/schema_test.yml
@@ -418,6 +418,36 @@ unions:
       message: "UNION type \"Baz\" must be OBJECT."
       locations: [{line: 1, column: 7}]
 
+  - name: unions of pure type extensions are valid
+    input: |
+
+      type Review {
+          body: String!
+          author: User! @provides(fields: "username")
+          product: Product!
+      }
+
+      extend type User @key(fields: "id") {
+        id: ID! @external
+        reviews: [Review]
+      }
+
+      extend type Product @key(fields: "upc") {
+        upc: String! @external
+        reviews: [Review]
+      }
+
+      union Foo = User | Product
+      scalar _Any
+      scalar _FieldSet
+      directive @external on FIELD_DEFINITION
+      directive @requires(fields: _FieldSet!) on FIELD_DEFINITION
+      directive @provides(fields: _FieldSet!) on FIELD_DEFINITION
+      directive @key(fields: _FieldSet!) on OBJECT | INTERFACE
+      directive @extends on OBJECT
+
+
+
 type extensions:
   - name: can extend non existant types
     input: |
diff --git a/validator/validator_test.go b/validator/validator_test.go
new file mode 100644
index 00000000..b8b77bdd
--- /dev/null
+++ b/validator/validator_test.go
@@ -0,0 +1,41 @@
+package validator_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/vektah/gqlparser"
+	"github.com/vektah/gqlparser/ast"
+	"github.com/vektah/gqlparser/parser"
+	"github.com/vektah/gqlparser/validator"
+)
+
+func TestExtendingNonExistantTypes(t *testing.T) {
+	s := gqlparser.MustLoadSchema(
+		&ast.Source{Name: "graph/schema.graphqls", Input: `
+extend type User {
+    id: ID!
+}
+
+extend type Product {
+    upc: String!
+}
+
+union _Entity = Product | User
+
+extend type Query {
+	entity: _Entity
+}
+`, BuiltIn: false},
+	)
+
+	q, err := parser.ParseQuery(&ast.Source{Name: "ff", Input: `{
+		entity {
+		  ... on User {
+			id
+		  }
+		}
+	}`})
+	require.Nil(t, err)
+	require.Nil(t, validator.Validate(s, q))
+}