diff --git a/calamari_ocr/ocr/evaluator.py b/calamari_ocr/ocr/evaluator.py
index 57bf26bc..4c640b32 100644
--- a/calamari_ocr/ocr/evaluator.py
+++ b/calamari_ocr/ocr/evaluator.py
@@ -257,5 +257,5 @@ def evaluate(self, *, gt_data: Dict[str, str], pred_data: Dict[str, str]):
         )
 
         res = Evaluator.evaluate_single_list(out, True)
-        res["ids"] = gt_ids
+        res["ids"] = list(gt_ids)
         return res