trouble with diff custom textconv

[frameworker2019]

Form v2.16.1, git 2.50.1 fork instance.

Defining a driver like this:

[diff "ignorecase"]
    textconv = "C:/Users/xyz/AppData/Local/Fork/gitInstance/2.50.1/usr/bin/tr.exe" '[:upper:]' '[:lower:]'

is interpreted, but ends in "tr: extra operand" message.

[diff "ignorecase"]
    textconv = "C:/Users/xyz/AppData/Local/Fork/gitInstance/2.50.1/usr/bin/tr.exe" '[:upper:]'

... ends with "tr: missing operand" - ok, so tr.exe seems to be executed....

I've tried any variant single, double quotes, quoted slashes etc. - but I cant't get it working that the textconv-filter will be applied before displaying the file.