diff --git a/cpu/avr/dev/clock.c b/cpu/avr/dev/clock.c
index 2be477c38..8640d57e2 100644
--- a/cpu/avr/dev/clock.c
+++ b/cpu/avr/dev/clock.c
@@ -5,7 +5,7 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
 
-static clock_time_t count;
+static volatile clock_time_t count;
 
 /*---------------------------------------------------------------------------*/
 SIGNAL(SIG_OUTPUT_COMPARE0)