diff --git a/sys/include/xtimer.h b/sys/include/xtimer.h
index c65a82acfa..05c68270b9 100644
--- a/sys/include/xtimer.h
+++ b/sys/include/xtimer.h
@@ -180,7 +180,9 @@ void xtimer_usleep_until(uint32_t *last_wakeup, uint32_t usecs);
  * The mesage struct specified by msg parameter will not be copied, e.g., it
  * needs to point to valid memory until the message has been delivered.
  *
- * @param[in] timer         timer struct to work with
+ * @param[in] timer         timer struct to work with.
+ *                          Its xtimer_t::target and xtimer_t::long_target
+ *                          fields need to be initialized with 0 on first use.
  * @param[in] offset        microseconds from now
  * @param[in] msg           ptr to msg that will be sent
  * @param[in] target_pid    pid the message will be sent to
@@ -196,7 +198,9 @@ void xtimer_set_msg(xtimer_t *timer, uint32_t offset, msg_t *msg, kernel_pid_t t
  * The mesage struct specified by msg parameter will not be copied, e.g., it
  * needs to point to valid memory until the message has been delivered.
  *
- * @param[in] timer         timer struct to work with
+ * @param[in] timer         timer struct to work with.
+ *                          Its xtimer_t::target and xtimer_t::long_target
+ *                          fields need to be initialized with 0 on first use.
  * @param[in] offset        microseconds from now
  * @param[in] msg           ptr to msg that will be sent
  * @param[in] target_pid    pid the message will be sent to
@@ -209,7 +213,9 @@ void xtimer_set_msg64(xtimer_t *timer, uint64_t offset, msg_t *msg, kernel_pid_t
  * This function sets a timer that will wake up a thread when the timer has
  * expired.
  *
- * @param[in] timer         timer struct to work with
+ * @param[in] timer         timer struct to work with.
+ *                          Its xtimer_t::target and xtimer_t::long_target
+ *                          fields need to be initialized with 0 on first use
  * @param[in] offset        microseconds from now
  * @param[in] pid           pid of the thread that will be woken up
  */
@@ -221,7 +227,9 @@ void xtimer_set_wakeup(xtimer_t *timer, uint32_t offset, kernel_pid_t pid);
  * This function sets a timer that will wake up a thread when the timer has
  * expired.
  *
- * @param[in] timer         timer struct to work with
+ * @param[in] timer         timer struct to work with.
+ *                          Its xtimer_t::target and xtimer_t::long_target
+ *                          fields need to be initialized with 0 on first use
  * @param[in] offset        microseconds from now
  * @param[in] pid           pid of the thread that will be woken up
  */
@@ -239,7 +247,9 @@ void xtimer_set_wakeup64(xtimer_t *timer, uint64_t offset, kernel_pid_t pid);
  * context (unless offset < XTIMER_BACKOFF). DON'T USE THIS FUNCTION unless you
  * know *exactly* what that means.
  *
- * @param[in] timer     the timer structure to use
+ * @param[in] timer     the timer structure to use.
+ *                      Its xtimer_t::target and xtimer_t::long_target
+ *                      fields need to be initialized with 0 on first use
  * @param[in] offset    time in microseconds from now specifying that timer's
  *                      callback's execution time
  */