TYPO3 Core

Having the typoscript below in TYPO3 v. 8.7.22 gives me what I want - a red box with white text in the middle....

20 = IMAGE
20{
    file = GIFBUILDER
    file{
        XY = 400,400
        backColor = #FF0000

        10 = BOX
        10{
            dimensions = 0,0,400,400
            color = #FF0000
        }

        20 = TEXT
        20{
            text = Testing...
            fontColor = #FFFFFF
            fontSize = 40
            niceText = 1
            align = center
            offset = 0,200
        }
    }
}

.... - having the exact same typoscript in TYPO3v. 9.5.3 gives me:

Argument 1 passed to TYPO3\CMS\Frontend\Resource\FilePathSanitizer::sanitize() must be of the type string, null given, called in /var/www/typo3_source/typo3_src-9.5.3/typo3/sysext/frontend/Classes/Imaging/GifBuilder.php on line 686