diff --git a/sys/include/net/gcoap.h b/sys/include/net/gcoap.h
index 22430d9afa568c18ce4c63cd574213047df155b1..1059d656cead2bac09f8c7dcd2825c7543e9a339 100644
--- a/sys/include/net/gcoap.h
+++ b/sys/include/net/gcoap.h
@@ -279,6 +279,8 @@ extern "C" {
 /**
  * @ingroup net_gcoap_conf
  * @brief   Length in bytes for a token
+ *
+ * Value must be in the range 0 to @ref GCOAP_TOKENLEN_MAX.
  */
 #ifndef GCOAP_TOKENLEN
 #define GCOAP_TOKENLEN          (2)