diff --git a/TCAP/asn_src/ITU/TC.asn b/TCAP/asn_src/ITU/TC.asn
index b002b6f..34aca66 100644
--- a/TCAP/asn_src/ITU/TC.asn
+++ b/TCAP/asn_src/ITU/TC.asn
@@ -32,7 +32,7 @@ Invoke ::= SEQUENCE {
 		absent	[1] IMPLICIT NULL
 	} OPTIONAL,
 	opcode Code,
-	argument Param
+	argument Param OPTIONAL
 }
 
 ReturnResult ::= SEQUENCE {