diff --git a/libserialport.h.in b/libserialport.h.in
index 49697a9..1dd4715 100644
--- a/libserialport.h.in
+++ b/libserialport.h.in
@@ -426,7 +426,7 @@ char *sp_get_port_description(struct sp_port *port);
  *
  * @since 0.1.1
  */
-enum sp_transport sp_get_port_transport(struct sp_port *port);
+enum sp_transport sp_get_port_transport(const struct sp_port *port);
 
 /**
  * Get the USB bus number and address on bus of a USB serial adapter port.
diff --git a/serialport.c b/serialport.c
index 9826df4..272bdf6 100644
--- a/serialport.c
+++ b/serialport.c
@@ -136,7 +136,7 @@ SP_API char *sp_get_port_description(struct sp_port *port)
 	RETURN_STRING(port->description);
 }
 
-SP_API enum sp_transport sp_get_port_transport(struct sp_port *port)
+SP_API enum sp_transport sp_get_port_transport(const struct sp_port *port)
 {
 	TRACE("%p", port);