diff --git a/scripts/prompt_matrix.py b/scripts/prompt_matrix.py
index e49c9b205..fd314b550 100644
--- a/scripts/prompt_matrix.py
+++ b/scripts/prompt_matrix.py
@@ -82,6 +82,6 @@ class Script(scripts.Script):
         processed.images.insert(0, grid)
 
         if opts.grid_save:
-            images.save_image(processed.images[0], p.outpath_grids, "prompt_matrix", prompt=original_prompt, seed=processed.seed, grid=True, p=p)
+            images.save_image(processed.images[0], p.outpath_grids, "prompt_matrix", extension=opts.grid_format, prompt=original_prompt, seed=processed.seed, grid=True, p=p)
 
         return processed