diff --git a/doc/reference.qbk b/doc/reference.qbk
index 7ce5487..73dd509 100644
--- a/doc/reference.qbk
+++ b/doc/reference.qbk
@@ -155,6 +155,7 @@
 
     unary_postfix_op  := primary_expr
                         | ( primary_expr "[" expression "]" )
+                        | ( primary_expr "()" )
                         | ( primary_expr "(" expression ( "," expression )* ")" )
                         | ( primary_expr "++" )
                         | ( primary_expr "--" )