diff --git a/tlv11.c b/tlv11.c
index c609f4e..5697c1b 100644
--- a/tlv11.c
+++ b/tlv11.c
@@ -109,7 +109,7 @@ uint8_t *tlv2str(struct tlv *tlv) {
     return s;
 }
 
-uint8_t *tlv2buf(uint8_t *p, struct tlv *tlv) {
+uint8_t *tlv2buf(uint8_t *p, const struct tlv *tlv) {
     *p++ = tlv->t;
     *p++ = tlv->l;
     if (tlv->l) {
diff --git a/tlv11.h b/tlv11.h
index d85f1fa..301768e 100644
--- a/tlv11.h
+++ b/tlv11.h
@@ -20,7 +20,7 @@ struct list *copytlvlist(struct list *);
 void freetlvlist(struct list *);
 void rmtlv(struct list *, uint8_t);
 uint8_t *tlv2str(struct tlv *tlv);
-uint8_t *tlv2buf(uint8_t *, struct tlv *tlv);
+uint8_t *tlv2buf(uint8_t *, const struct tlv *tlv);
 
 /* Local Variables: */
 /* c-file-style: "stroustrup" */