Index: tests/phpunit/includes/testcase.php
===================================================================
--- tests/phpunit/includes/testcase.php	(revision 37066)
+++ tests/phpunit/includes/testcase.php	(working copy)
@@ -292,6 +292,12 @@
 	}
 
 	function wp_die_handler( $message ) {
+		if ( function_exists( 'is_wp_error' ) && is_wp_error( $message ) ) {
+			$errors = $message->get_error_messages();
+			if ( 0 < count( $errors ) ) {
+				$message = join( ', ', $errors );
+			}
+		}
 		throw new WPDieException( $message );
 	}