diff --git a/module_usb_audio/endpoint0/vendorrequests.h b/module_usb_audio/endpoint0/vendorrequests.h
index 680464ba..e571ba92 100644
--- a/module_usb_audio/endpoint0/vendorrequests.h
+++ b/module_usb_audio/endpoint0/vendorrequests.h
@@ -1,6 +1,8 @@
 #ifndef _VENDORREQUESTS_H_
 #define _VENDORREQUESTS_H_
 
+#include <xccompat.h>
+
 /* Functions that handle vustomer vendor requests.
  *
  * THESE NEED IMPLEMENTING FOR A SPECIFIC DESIGN
@@ -10,7 +12,9 @@
  * */
 
 int VendorAudioRequests(XUD_ep ep0_out, XUD_ep ep0_in, unsigned char bRequest, unsigned char cs, unsigned char cn,
-    unsigned short unitId, unsigned char direction, chanend c_audioControl, chanend ?c_mix_ctl, chanend ?c_clk_ctL);
+    unsigned short unitId, unsigned char direction, chanend c_audioControl, 
+    NULLABLE_RESOURCE(chanend, c_mix_ctl), 
+    NULLABLE_RESOURCE(chanend, c_clk_ctL));
 
 #endif