add const
diff --git a/skeletons/BMPString.c b/skeletons/BMPString.c
index 649882e..80a3f8b 100644
--- a/skeletons/BMPString.c
+++ b/skeletons/BMPString.c
@@ -128,7 +128,7 @@
 }
 
 asn_dec_rval_t
-BMPString_decode_xer(asn_codec_ctx_t *opt_codec_ctx,
+BMPString_decode_xer(const asn_codec_ctx_t *opt_codec_ctx,
 	asn_TYPE_descriptor_t *td, void **sptr,
 		const char *opt_mname, const void *buf_ptr, size_t size) {
 	asn_dec_rval_t rc;