diff --git a/testsuite/MDAnalysisTests/util.py b/testsuite/MDAnalysisTests/util.py
index a1493dd4ff..88631bdcff 100644
--- a/testsuite/MDAnalysisTests/util.py
+++ b/testsuite/MDAnalysisTests/util.py
@@ -233,6 +233,9 @@ def _warn(self, message, category=None, *args, **kwargs):
         if isinstance(message, Warning):
             self._captured_categories.append(message.__class__)
         else:
+            # as follows Python documentation at
+            # https://docs.python.org/3/library/warnings.html#warnings.warn
+            # if category is None, the default UserWarning is used
             if category is None:
                 category = UserWarning
             self._captured_categories.append(category)