diff --git a/src/CrawlerDetect.php b/src/CrawlerDetect.php
index 777bdc3..89287cb 100644
--- a/src/CrawlerDetect.php
+++ b/src/CrawlerDetect.php
@@ -165,7 +165,7 @@ public function isCrawler($userAgent = null)
         $agent = trim(preg_replace(
             "/{$this->compiledExclusions}/i",
             '',
-            $userAgent ?: $this->userAgent
+            $userAgent ?: $this->userAgent ?: ''
         ));
 
         if ($agent === '') {