diff --git a/tests/scripts/analyze_outcomes.py b/tests/scripts/analyze_outcomes.py
index 5f2e378..d0b72a8 100755
--- a/tests/scripts/analyze_outcomes.py
+++ b/tests/scripts/analyze_outcomes.py
@@ -35,7 +35,7 @@
 
     @staticmethod
     def _print_line(fmt, *args, **kwargs):
-        sys.stderr.write(fmt + '\n', *args, **kwargs)
+        sys.stderr.write((fmt + '\n').format(*args, **kwargs))
 
 class TestCaseOutcomes:
     """The outcomes of one test case across many configurations."""