diff --git a/include/lib/tftf_lib.h b/include/lib/tftf_lib.h
index 40955d3..58112b4 100644
--- a/include/lib/tftf_lib.h
+++ b/include/lib/tftf_lib.h
@@ -131,7 +131,7 @@
  */
 typedef struct {
 	/* Function identifier. Identifies which function is being invoked. */
-	uint32_t	arg0;
+	uint32_t	fid;
 
 	u_register_t	arg1;
 	u_register_t	arg2;