diff --git a/slim/prune/prune.py b/slim/prune/prune.py
index 6f6dcc4cad5de9aede1c67b4de2e296815a646ec..2b7815059ea240f1aaaec645deaea116f37d386e 100644
--- a/slim/prune/prune.py
+++ b/slim/prune/prune.py
@@ -307,6 +307,9 @@ def main():
 
             if FLAGS.eval:
                 # evaluation
+                resolution = None
+                if 'Mask' in cfg.architecture:
+                    resolution = model.mask_head.resolution
                 results = eval_run(
                     exe,
                     compiled_eval_prog,
@@ -314,10 +317,8 @@ def main():
                     eval_keys,
                     eval_values,
                     eval_cls,
-                    cfg=cfg)
-                resolution = None
-                if 'mask' in results[0]:
-                    resolution = model.mask_head.resolution
+                    cfg=cfg,
+                    resolution=resolution)
                 box_ap_stats = eval_results(
                     results,
                     cfg.metric,