diff --git a/module_usb_audio/endpoint0/descriptors.h b/module_usb_audio/endpoint0/descriptors.h
index 06da7df6..a12937b9 100644
--- a/module_usb_audio/endpoint0/descriptors.h
+++ b/module_usb_audio/endpoint0/descriptors.h
@@ -263,10 +263,12 @@ enum {
     INPUT_INTERFACE_STRING_INDEX,
     INPUT_INTERFACE_LAST_STRING_INDEX = INPUT_INTERFACE_STRING_INDEX + NUM_USB_CHAN_IN - 1,
 #ifdef MIXER
+#if MAX_MIX_COUNT > 0
     MIXER_STRING_INDEX,
 #endif
+#endif
 #ifdef IAP
-    IAP_INTERFACE_STRING_INDEX,
+    IAP_INTERFACE_STRING_INDEX = INPUT_INTERFACE_LAST_STRING_INDEX + MAX_MIX_COUNT + 1,
 #endif
 };