diff --git a/parser.go b/parser.go
index 1f801a2..1ca0d1b 100644
--- a/parser.go
+++ b/parser.go
@@ -441,6 +441,9 @@ func (parser *parser) parseGeneral(scanner *scanner, ctx parserContext, termSymb
 				tk.Sym = SymChangeSign
 			} else if tk.Sym == SymPlus {
 				tk.Sym = SymUnchangeSign
+			} else if tk.IsSymbol(SymExclamation) {
+				err = tk.Errorf("postfix opertor %q requires an operand on its left", tk)
+				break
 			}
 			firstToken = false
 		}