diff --git a/wp-includes/class.wp-styles.php b/wp-includes/class.wp-styles.php
index 83b4addbc0..97acdf804a 100644
--- a/wp-includes/class.wp-styles.php
+++ b/wp-includes/class.wp-styles.php
@@ -100,7 +100,7 @@ class WP_Styles extends WP_Dependencies {
 	}
 
 	function _css_href( $src, $ver, $handle ) {
-		if ( !preg_match('|^https?://|', $src) && ! ( $this->content_url && 0 === strpos($src, $this->content_url) ) ) {
+		if ( !is_bool($src) && !preg_match('|^https?://|', $src) && ! ( $this->content_url && 0 === strpos($src, $this->content_url) ) ) {
 			$src = $this->base_url . $src;
 		}