diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index e53b694..d699ed0 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -983,7 +983,7 @@
             elif sig == signal.CTRL_BREAK_EVENT:
                 os.kill(self.pid, signal.CTRL_BREAK_EVENT)
             else:
-                raise ValueError("Unsupported signal")
+                raise ValueError("Unsupported signal: {}".format(sig))
 
         def terminate(self):
             """Terminates the process