diff --git a/include/osmocom/gprs/gprs_ns2.h b/include/osmocom/gprs/gprs_ns2.h
index cd2de96..6b88c72 100644
--- a/include/osmocom/gprs/gprs_ns2.h
+++ b/include/osmocom/gprs/gprs_ns2.h
@@ -153,7 +153,7 @@
 /* vty */
 int gprs_ns2_vty_init(struct gprs_ns2_inst *nsi);
 int gprs_ns2_vty_create();
-void gprs_ns2_vty_force_vc_mode(bool force, enum gprs_ns2_vc_mode mode, char *reason);
+void gprs_ns2_vty_force_vc_mode(bool force, enum gprs_ns2_vc_mode mode, const char *reason);
 
 
 /*! @} */