diff --git src/wp-includes/rest-api/class-wp-rest-response.php src/wp-includes/rest-api/class-wp-rest-response.php
index fed1a31..db80029 100644
--- src/wp-includes/rest-api/class-wp-rest-response.php
+++ src/wp-includes/rest-api/class-wp-rest-response.php
@@ -243,8 +243,12 @@ class WP_REST_Response extends WP_HTTP_Response {
 		$error = new WP_Error;
 
 		if ( is_array( $this->get_data() ) ) {
-			foreach ( $this->get_data() as $err ) {
-				$error->add( $err['code'], $err['message'], $err['data'] );
+			$data = $this->get_data();
+			$error->add( $data['code'], $data['message'], $data['data'] );
+			if ( ! empty( $data['additional_errors'] ) ) {
+				foreach( $data['additional_errors'] as $err ) {
+					$error->add( $err['code'], $err['message'], $err['data'] );
+				}
 			}
 		} else {
 			$error->add( $this->get_status(), '', array( 'status' => $this->get_status() ) );
diff --git tests/phpunit/tests/rest-api/rest-server.php tests/phpunit/tests/rest-api/rest-server.php
index 93c3a27..248ba56 100644
--- tests/phpunit/tests/rest-api/rest-server.php
+++ tests/phpunit/tests/rest-api/rest-server.php
@@ -282,6 +282,27 @@ class Tests_REST_Server extends WP_Test_REST_TestCase {
 		$this->assertEquals( $message, $data['message'] );
 	}
 
+	public function test_error_to_response_to_error() {
+		$code     = 'wp-api-test-error';
+		$message  = 'Test error message for the API';
+		$code2    = 'wp-api-test-error-2';
+		$message2 = 'Another test message';
+		$error   = new WP_Error( $code, $message, array( 'status' => 400 ) );
+		$error->add( $code2, $message2, array( 'status' => 403 ) );
+
+		$response = $this->server->error_to_response( $error );
+		$this->assertInstanceOf( 'WP_REST_Response', $response );
+
+		$this->assertEquals( 400, $response->get_status() );
+
+		$error = $response->as_error();
+		$this->assertInstanceOf( 'WP_Error', $error );
+		$this->assertEquals( $code, $error->get_error_code() );
+		$this->assertEquals( $message, $error->get_error_message() );
+		$this->assertEquals( $message2, $error->errors[ $code2 ][0] );
+		$this->assertEquals( array( 'status' => 403 ), $error->error_data[ $code2 ] );
+	}
+
 	public function test_rest_error() {
 		$data = array(
 			'code'    => 'wp-api-test-error',