diff --git a/parser.go b/parser.go
index 717293c..e73da2a 100644
--- a/parser.go
+++ b/parser.go
@@ -25,7 +25,7 @@ func (self *parser) parseFunction(scanner *scanner, tk *Token) (tree *term, err
 	name, _ := tk.Value.(string)
 	funcObj := self.ctx.GetFuncInfo(name)
 	if funcObj == nil {
-		err = fmt.Errorf("unknown function %q", name)
+		err = fmt.Errorf("unknown function %s()", name)
 		return
 	}
 	maxArgs := funcObj.MaxArgs()