diff --git a/src/Prometheus/Collector.php b/src/Prometheus/Collector.php
index edc5e80..5de1d8e 100644
--- a/src/Prometheus/Collector.php
+++ b/src/Prometheus/Collector.php
@@ -69,7 +69,7 @@ public function getKey()
     protected function assertLabelsAreDefinedCorrectly($labels)
     {
         if (count($labels) != count($this->labels)) {
-            throw new \InvalidArgumentException(sprintf('Labels are not defined correctly: ', print_r($labels, true)));
+            throw new \InvalidArgumentException(sprintf('Labels are not defined correctly: %s', print_r($labels, true)));
         }
     }
 }