Do not split the arguments if any of them is undefined

index.js

@@ -104,15 +104,21 @@ function arg(
 		}
 
 		if (wholeArg.length > 1 && wholeArg[0] === '-') {
-			/* eslint-disable operator-linebreak */
-			const separatedArguments =
-				wholeArg[1] === '-' || wholeArg.length === 2
-					? [wholeArg]
-					: wholeArg
-							.slice(1)
-							.split('')
-							.map((a) => `-${a}`);
-			/* eslint-enable operator-linebreak */
+			let separatedArguments = [];
+			if (wholeArg[1] === '-' || wholeArg.length === 2) {
+				separatedArguments.push(wholeArg);
+			} else {
+				for (const char of wholeArg.slice(1)) {
+					separatedArguments.push(`-${char}`);
+				}
+			}
+
+			const allArgumentsExist = separatedArguments.every(
+				(flag) => flag in handlers
+			);
+			if (!allArgumentsExist && permissive) {
+				separatedArguments = [wholeArg];
+			}
 
 			for (let j = 0; j < separatedArguments.length; j++) {
 				const arg = separatedArguments[j];

test.js

@@ -481,8 +481,27 @@ test('should stop parsing early with permissive', () => {
 	);
 
 	expect(result).to.deep.equal({
-		_: ['-v', '--foo', 'bar'],
-		'-d': 2
+		_: ['-dvd', '--foo', 'bar']
+	});
+});
+
+test('should not split unknown arguments with permissive', () => {
+	const argv = ['-dvd', '--foo', 'bar'];
+
+	const result = arg(
+		{
+			'-d': arg.COUNT,
+			'--foo': Boolean
+		},
+		{
+			argv,
+			permissive: true
+		}
+	);
+
+	expect(result).to.deep.equal({
+		_: ['-dvd', 'bar'],
+		'--foo': true
 	});
 });