diff --git a/gen_img.py b/gen_img.py
index a6c616e9..119aaa4f 100644
--- a/gen_img.py
+++ b/gen_img.py
@@ -2812,7 +2812,7 @@ def main(args):
                 # override filename to add index number if more than one image per prompt
                 if filename is not None and (args.images_per_prompt >  1 or len(raw_prompts) > 1):
                     fileext = os.path.splitext(filename)
-                    filename = fileext[0] + "_%s" + fileext[1] % pi
+                    filename = fileext[0] + "_%s" % pi + fileext[1]
                     
                 # override Deep Shrink    
                 if ds_depth_1 is not None: