diff --git a/vlib/flag/flag.v b/vlib/flag/flag.v
index ef317f367f..0dbfb496ed 100644
--- a/vlib/flag/flag.v
+++ b/vlib/flag/flag.v
@@ -404,17 +404,15 @@ pub fn (fs FlagParser) usage() string {
 	return use.replace('- ,', '   ')
 }
 
-// finalize argument parsing -> call after all arguments are defined
-//
-// all remaining arguments are returned in the same order they are defined on
-// command line
-//
-// if additional flag are found (things starting with '--') an error is returned
-// error handling is up to the application developer
+// finalize - return all remaining arguments (non options).
+// Call .finalize() after all arguments are defined.
+// The remaining arguments are returned in the same order they are
+// defined on the command line. If additional flags are found, i.e.
+// (things starting with '--' or '-'), it returns an error.
 pub fn (fs FlagParser) finalize() ?[]string {
 	for a in fs.args {
-		if a.len >= 2 && a[..2] == '--' {
-			return error("Unknown argument \'${a[2..]}\'")
+		if (a.len >= 2 && a[..2] == '--') || (a.len == 2 && a[0] == `-`) {
+			return error('Unknown flag `$a`')
 		}
 	}
 	if fs.args.len < fs.min_free_args && fs.min_free_args > 0 {
diff --git a/vlib/flag/flag_test.v b/vlib/flag/flag_test.v
index 34bbb0a5bf..c875989869 100644
--- a/vlib/flag/flag_test.v
+++ b/vlib/flag/flag_test.v
@@ -133,11 +133,21 @@ fn test_finalize_returns_none_flag_arguments_ordered() {
 	}
 }
 
-fn test_finalize_returns_error_for_unknown_flags() {
+fn test_finalize_returns_error_for_unknown_flags_long() {
 	mut fp := flag.new_flag_parser(['--known', '--unknown'])
 	fp.bool('known', 0, false, '')
 	finalized := fp.finalize() or {
-		assert err.msg == "Unknown argument 'unknown'"
+		assert err.msg == 'Unknown flag `--unknown`'
+		return
+	}
+	assert finalized.len < 0 // expect error to be returned
+}
+
+fn test_finalize_returns_error_for_unknown_flags_short() {
+	mut fp := flag.new_flag_parser(['--known', '-x'])
+	fp.bool('known', 0, false, '')
+	finalized := fp.finalize() or {
+		assert err.msg == 'Unknown flag `-x`'
 		return
 	}
 	assert finalized.len < 0 // expect error to be returned