diff --git a/run_wml_tests b/run_wml_tests
index 4f1d6ebd71e0..cd2f2439f04e 100755
--- a/run_wml_tests
+++ b/run_wml_tests
@@ -180,7 +180,7 @@ class WesnothRunner:
             res = run_with_rerun_for_sdl_video(args, timeout)
         except subprocess.TimeoutExpired as t:
             print("Timed out (killed by Python timeout implementation)")
-            res = subprocess.CompletedProcess(args, UnitTestResult.TIMEOUT.value, t.output)
+            res = subprocess.CompletedProcess(args, UnitTestResult.TIMEOUT.value, t.output or b'')
         if self.verbose > 0:
             print(res.stdout.decode('utf-8'))
             if self.verbose > 1: