diff --git a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
index 6a68c67431..e3ffc421ce 100755
--- a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
+++ b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
@@ -525,6 +525,7 @@ def write_I2C_config(f):
 def write_PWM_config(f):
     '''write PWM config defines'''
     rc_in = None
+    alarm = None
     pwm_out = []
     pwm_timers = []
     for l in bylabel.keys():