diff --git a/library/Opus/Doi/DoiManager.php b/library/Opus/Doi/DoiManager.php
index 23a58772..443ce4a2 100644
--- a/library/Opus/Doi/DoiManager.php
+++ b/library/Opus/Doi/DoiManager.php
@@ -59,7 +59,6 @@
 use function file_put_contents;
 use function get_class;
 use function is_dir;
-use function is_numeric;
 use function is_string;
 use function ltrim;
 use function mkdir;