diff --git a/src/_pytest/junitxml.py b/src/_pytest/junitxml.py
index ba0f48ed39d..fa86059aebd 100644
--- a/src/_pytest/junitxml.py
+++ b/src/_pytest/junitxml.py
@@ -207,6 +207,8 @@ def write_captured_output(self, report):
             else:
                 content = content_err
 
+            # XXX: flaky in test suite, trying to figure out why.
+            assert content, repr([content, content_log, content_err])
             if content:
                 tag = getattr(Junit, "system-err")
                 self.append(tag(bin_xml_escape(content)))