diff --git a/libraries/AP_PeriodicProcess/AP_TimerProcess.cpp b/libraries/AP_PeriodicProcess/AP_TimerProcess.cpp
index 38e1edbfb7..5ba103de2f 100644
--- a/libraries/AP_PeriodicProcess/AP_TimerProcess.cpp
+++ b/libraries/AP_PeriodicProcess/AP_TimerProcess.cpp
@@ -49,8 +49,10 @@ void AP_TimerProcess::register_process(ap_procedure proc)
     for (uint8_t i=0; i<_pidx; i++) {
         if (_proc[i] == proc) return;
     }
+    cli();
     if (_pidx < AP_TIMERPROCESS_MAX_PROCS)
         _proc[_pidx++] = proc;
+    sei();
 }
 
 void AP_TimerProcess::set_failsafe(ap_procedure proc)