diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index 6a49c39..cf8bed9 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -298,7 +298,7 @@
  *                  cipher sub-context.
  *
  * \param ctx       The GCM context to clear. If this is \c NULL, the call has
- *                  no effect.
+ *                  no effect. Otherwise, this must be initialized.
  */
 void mbedtls_gcm_free( mbedtls_gcm_context *ctx );