diff --git a/Lib/test/libregrtest/run_workers.py b/Lib/test/libregrtest/run_workers.py
index 1a14177e51ab7d..dff631f09c5abf 100644
--- a/Lib/test/libregrtest/run_workers.py
+++ b/Lib/test/libregrtest/run_workers.py
@@ -433,7 +433,8 @@ def run(self) -> None:
     def _wait_completed(self) -> None:
         popen = self._popen
         # only needed for mypy:
-        assert popen is not None, "Should never call `._popen` before `.run()`"
+        if popen is None:
+            raise ValueError("Should never call `._popen` before `.run()`")
 
         try:
             popen.wait(WAIT_COMPLETED_TIMEOUT)