diff --git a/src/Collectors/Backtracer.php b/src/Collectors/Backtracer.php
index d31c9c0..c5989cd 100644
--- a/src/Collectors/Backtracer.php
+++ b/src/Collectors/Backtracer.php
@@ -30,7 +30,7 @@ public function getCallerClass(array $backtrace): string
     protected function parseTrace($index, array $trace)
     {
         if (isset($trace['class']) && !$this->isExcludedClass($trace['class'])) {
-            return $trace['class'] . ':' . ($trace['line'] ?? '?');
+            return $trace['class'] . ':' . ($trace['line'] ?? '');
         }
 
         return '';