diff --git a/src/CSS.php b/src/CSS.php
index e5a4669..a9a70a0 100644
--- a/src/CSS.php
+++ b/src/CSS.php
@@ -525,7 +525,7 @@ protected function shortenColors($content)
         );
 
         return preg_replace_callback(
-            '/(?<=[: ])('.implode(array_keys($colors), '|').')(?=[; }])/i',
+            '/(?<=[: ])('.implode('|', array_keys($colors)).')(?=[; }])/i',
             function ($match) use ($colors) {
                 return $colors[strtoupper($match[0])];
             },