diff --git a/openmc/universe.py b/openmc/universe.py
index bdd6e6ec9..9c93e6da1 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -361,7 +361,7 @@ class Universe(UniverseBase):
             img_path = Path(tmpdir) / f'plot_{plot.id}.png'
             if not img_path.is_file():
                 img_path = img_path.with_suffix('.ppm')
-            img = mpimg.imread(img_path)
+            img = mpimg.imread(str(img_path))
 
             # Create a figure sized such that the size of the axes within
             # exactly matches the number of pixels specified