diff --git a/src/gsm/gsm48.c b/src/gsm/gsm48.c
index fc3b973..ae9a1cf 100644
--- a/src/gsm/gsm48.c
+++ b/src/gsm/gsm48.c
@@ -604,6 +604,7 @@
 /*! Generate TS 24.008 §10.5.1.4 Mobile ID
  *  \param[out] buf Caller-provided output buffer
  *  \param[in] id Identity to be encoded
+ *  \param[in] mi_type Type of identity (e.g. GSM_MI_TYPE_TMSI)
  *  \returns number of bytes used in \a buf */
 uint8_t gsm48_generate_mid(uint8_t *buf, const char *id, uint8_t mi_type)
 {