add const
diff --git a/skeletons/BIT_STRING_oer.c b/skeletons/BIT_STRING_oer.c
index 9886646..a470e09 100644
--- a/skeletons/BIT_STRING_oer.c
+++ b/skeletons/BIT_STRING_oer.c
@@ -10,7 +10,7 @@
 #include <errno.h>
 
 asn_dec_rval_t
-BIT_STRING_decode_oer(asn_codec_ctx_t *opt_codec_ctx, asn_TYPE_descriptor_t *td,
+BIT_STRING_decode_oer(const asn_codec_ctx_t *opt_codec_ctx, asn_TYPE_descriptor_t *td,
                       const asn_oer_constraints_t *constraints, void **sptr,
                       const void *ptr, size_t size) {
     BIT_STRING_t *st = (BIT_STRING_t *)*sptr;