diff --git a/cpu/nrf51/periph/i2c.c b/cpu/nrf51/periph/i2c.c
index d532476111..68c10b7a46 100644
--- a/cpu/nrf51/periph/i2c.c
+++ b/cpu/nrf51/periph/i2c.c
@@ -28,6 +28,7 @@
  *
  * @}
  */
+#include <assert.h>
 #include <errno.h>
 #include "cpu.h"
 #include "mutex.h"
@@ -139,12 +140,11 @@ int i2c_acquire(i2c_t dev)
     return 0;
 }
 
-int i2c_release(i2c_t dev)
+void i2c_release(i2c_t dev)
 {
     assert(dev < I2C_NUMOF);
 
     mutex_unlock(&locks[dev]);
-    return 0;
 }
 
 int i2c_read_bytes(i2c_t dev, uint16_t address, void *data, size_t length,