I answered my own question by the way:

```bash
1) ArrayHelperTest::testInvert with data set "Case 1" (array(array('1000', '1500', '1750'), array('3000', '4000', '5000', '6000')), array('New', 'New', 'New', 'Used', 'Used', 'Used', 'Used'))
array_flip(): Can only flip STRING and INTEGER values!

/src/ArrayHelper.php:331
/Tests/ArrayHelperTest.php:1458

2) ArrayHelperTest::testInvert with data set "Case 2" (array(array(1000, 1500, 1750), array(2750, 3000, 4000, 5000, 6000), array(2000, 2500), array()), array('New', 'New', 'New', 'Used', 'Used', 'Used', 'Used', 'Used', 'Refurbished', 'Refurbished'))
array_flip(): Can only flip STRING and INTEGER values!

/src/ArrayHelper.php:331
/Tests/ArrayHelperTest.php:1458
```