diff --git a/flag.go b/flag.go
index 7b84e2cd..85c34a7c 100644
--- a/flag.go
+++ b/flag.go
@@ -916,6 +916,9 @@ func (f *FlagSet) parseLongArg(s string, args []string, fn parseFunc) (a []strin
 	}
 
 	err = fn(flag, value)
+	if err != nil {
+		f.failf(err.Error())
+	}
 	return
 }
 
@@ -966,6 +969,9 @@ func (f *FlagSet) parseSingleShortArg(shorthands string, args []string, fn parse
 	}
 
 	err = fn(flag, value)
+	if err != nil {
+		f.failf(err.Error())
+	}
 	return
 }