diff --git a/tools/lib/traceevent/event-parse.c b/tools/lib/traceevent/event-parse.c
index 6270ee2e9830..27088c55af1b 100644
--- a/tools/lib/traceevent/event-parse.c
+++ b/tools/lib/traceevent/event-parse.c
@@ -1719,6 +1719,7 @@ process_op(struct event_format *event, struct print_arg *arg, char **tok)
 		arg->op.left = left;
 		arg->op.prio = 0;
 
+		/* it will set arg->op.right */
 		type = process_cond(event, arg, tok);
 
 	} else if (strcmp(token, ">>") == 0 ||
@@ -1745,6 +1746,7 @@ process_op(struct event_format *event, struct print_arg *arg, char **tok)
 		arg->type = PRINT_OP;
 		arg->op.op = token;
 		arg->op.left = left;
+		arg->op.right = NULL;
 
 		if (set_op_prio(arg) == -1) {
 			event->flags |= EVENT_FL_FAILED;
@@ -1792,6 +1794,7 @@ process_op(struct event_format *event, struct print_arg *arg, char **tok)
 
 		arg->op.prio = 0;
 
+		/* it will set arg->op.right */
 		type = process_array(event, arg, tok);
 
 	} else {