diff --git a/config/scripts/smtpdummy b/config/scripts/smtpdummy
index f8cd0aa..adb224d 100755
--- a/config/scripts/smtpdummy
+++ b/config/scripts/smtpdummy
@@ -141,7 +141,10 @@ Input arguments:
         next = True
         while next:
             line = p.stdout.readline()
-            code = int(line[:3])
+            try:
+                code = int(line[:3])
+            except ValueError:
+                raise ValueError(f"Could not parse line: '{line.decode()}'")
             success = code < 400
             color = colorama.Fore.GREEN if success else colorama.Fore.RED
             if not args.quiet: