diff --git a/wp-includes/default-constants.php b/wp-includes/default-constants.php
index 4e5daabd4d..476580c931 100644
--- a/wp-includes/default-constants.php
+++ b/wp-includes/default-constants.php
@@ -39,8 +39,11 @@ function wp_initial_constants( ) {
 		$blog_id = 1;
 
 	// set memory limits.
-	if ( function_exists('memory_get_usage') && ( (int) @ini_get('memory_limit') < abs(intval(WP_MEMORY_LIMIT)) ) )
-		@ini_set('memory_limit', WP_MEMORY_LIMIT);
+	if ( function_exists( 'memory_get_usage' ) ) {
+		$current_limit = @ini_get( 'memory_limit' );
+		if ( -1 != $current_limit && ( -1 == WP_MEMORY_LIMIT || ( inval( $current_limit ) < abs( intval( WP_MEMORY_LIMIT ) ) ) ) )
+			@ini_set( 'memory_limit', WP_MEMORY_LIMIT );
+	}
 
 	if ( !defined('WP_CONTENT_DIR') )
 		define( 'WP_CONTENT_DIR', ABSPATH . 'wp-content' ); // no trailing slash, full paths only - WP_CONTENT_URL is defined further down