diff --git a/src/Cool/CoolRequest.php b/src/Cool/CoolRequest.php
index e9f357c8..89c03580 100644
--- a/src/Cool/CoolRequest.php
+++ b/src/Cool/CoolRequest.php
@@ -70,7 +70,7 @@ public function getWopiClientURL() {
             );
         }
 
-        $wopi_src = strval($this->getWopiSrcUrl($discovery_parsed, 'text/plain')[0]);
+        $wopi_src = (string) $this->getWopiSrcUrl($discovery_parsed, 'text/plain')[0];
         if (!$wopi_src) {
             throw new CoolRequestException(
                 'The requested mime type is not handled.',