diff --git a/references/detection/coco_eval.py b/references/detection/coco_eval.py
index 3460fa66456..d758a64a909 100644
--- a/references/detection/coco_eval.py
+++ b/references/detection/coco_eval.py
@@ -109,7 +109,7 @@ def prepare_for_coco_segmentation(self, predictions):
             labels = prediction["labels"].tolist()
 
             rles = [
-                mask_util.encode(np.array(mask[0, :, :, np.newaxis], order="F"))[0]
+                mask_util.encode(np.array(mask[0, :, :, np.newaxis], dtype=np.uint8, order="F"))[0]
                 for mask in masks
             ]
             for rle in rles: