diff --git a/platformio/test/runners/readers/serial.py b/platformio/test/runners/readers/serial.py
index 29916915..a9732d7f 100644
--- a/platformio/test/runners/readers/serial.py
+++ b/platformio/test/runners/readers/serial.py
@@ -46,7 +46,7 @@ class SerialTestOutputReader:
             ser.open()
         except serial.SerialException as exc:
             click.secho(str(exc), fg="red", err=True)
-            return None
+            return
 
         if not self.test_runner.options.no_reset:
             ser.flushInput()