diff --git a/model_server/accessors.py b/model_server/accessors.py
index 62df189d1f8157ffae3f415035574aa3ef81c697..4f122fb13d8022255f5866daef4646dd3940a00e 100644
--- a/model_server/accessors.py
+++ b/model_server/accessors.py
@@ -190,7 +190,7 @@ def write_accessor_data_to_file(fpath: Path, accessor: GenericImageDataAccessor,
         )
         if accessor.is_mask():
             if accessor.dtype == 'bool':
-                data = zcyx * 255
+                data = (zcyx * 255).astype('uint8')
             else:
                 data = zcyx.astype('uint8')
             tifffile.imwrite(fpath, data, imagej=True)