diff --git a/include/coap2/coap_session.h b/include/coap2/coap_session.h
index 15140378..1fddebc8 100644
--- a/include/coap2/coap_session.h
+++ b/include/coap2/coap_session.h
@@ -160,7 +160,7 @@ void coap_session_set_mtu(coap_session_t *session, unsigned mtu);
  * @param session The CoAP session.
  * @return maximum PDU size, not including header (but including token).
  */
-size_t coap_session_max_pdu_size(coap_session_t *session);
+size_t coap_session_max_pdu_size(const coap_session_t *session);
 
 /**
 * Creates a new client session to the designated server.
diff --git a/src/coap_session.c b/src/coap_session.c
index 49b1517a..9fc52e0e 100644
--- a/src/coap_session.c
+++ b/src/coap_session.c
@@ -186,7 +186,7 @@ void coap_session_free(coap_session_t *session) {
   coap_free_type(COAP_SESSION, session);
 }
 
-size_t coap_session_max_pdu_size(coap_session_t *session) {
+size_t coap_session_max_pdu_size(const coap_session_t *session) {
   size_t max_with_header = (size_t)(session->mtu - session->tls_overhead);
   if (COAP_PROTO_NOT_RELIABLE(session->proto))
     return max_with_header > 4 ? max_with_header - 4 : 0;