diff --git a/lib/rpn_msr/proposal_target_layer_tf.py b/lib/rpn_msr/proposal_target_layer_tf.py
index b092f45..0dc063c 100644
--- a/lib/rpn_msr/proposal_target_layer_tf.py
+++ b/lib/rpn_msr/proposal_target_layer_tf.py
@@ -165,7 +165,7 @@ def _sample_rois(all_rois, gt_boxes, gt_ishard, dontcare_areas, fg_rois_per_imag
     # Compute number of background RoIs to take from this image (guarding
     # against there being fewer than desired)
     bg_rois_per_this_image = rois_per_image - fg_rois_per_this_image
-    bg_rois_per_this_image = min(bg_rois_per_this_image, bg_inds.size)
+    bg_rois_per_this_image = int(min(bg_rois_per_this_image, bg_inds.size))
     # Sample background regions without replacement
     if bg_inds.size > 0:
         bg_inds = npr.choice(bg_inds, size=bg_rois_per_this_image, replace=False)