diff --git a/parser/tree-tostring.go b/parser/tree-tostring.go
index e9d5b0e..f3253ae 100644
--- a/parser/tree-tostring.go
+++ b/parser/tree-tostring.go
@@ -143,8 +143,8 @@ func (phrase *Phrase) ToString (indent int, breakLine bool) (output string) {
 
 func (argument *Argument) ToString (indent int, breakLine bool) (output string) {
 	if !breakLine { indent = 0 }
-	if argument.value == nil {
-		output += "NIL ARGUMENT"
+	if argument.kind == ArgumentKindNil {
+		output += "NIL-ARGUMENT"
 		if breakLine { output += "\n" }
 		return
 	}