diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
index 8f52a48097ff..2f4fdf18c08d 100644
--- a/lib/matplotlib/testing/compare.py
+++ b/lib/matplotlib/testing/compare.py
@@ -204,9 +204,9 @@ def __call__(self, orig, dest):
             # Inkscape's output is not localized but gtk's is, so the output
             # stream probably has a mixed encoding.  Using the filesystem
             # encoding should at least get the filenames right...
-            self._stderr.seek(0)
+            self._proc.stderr.seek(0)
             raise ImageComparisonFailure(
-                self._stderr.read().decode(
+                self._proc.stderr.read().decode(
                     sys.getfilesystemencoding(), "replace")) from err