JSON Parse error when viewing custom report (Report Builder Type)

I created a custom report based on Timesheet / Projects.
When I try to view the report i get a json parse error and the page gets stuck.

[Error] SyntaxError: JSON Parse error: Unexpected identifier "undefined"
    parse (report.min.js:1:613)
    (anonymous function) (report.min.js:1:613)
    with_doc (desk.min.js:538:199)
    (anonymous function) (report.min.js:1:525)
    callback (desk.min.js:535)
    callback (desk.min.js:130)
    (anonymous function) (desk.min.js:132:149)
    (anonymous function) (desk.min.js:137:105)
    i (jquery.min.js:2:27157)
    fireWith (jquery.min.js:2:27916)
    z (jquery.min.js:4:12071)
    (anonymous function) (jquery.min.js:4:15621)
[Error] TypeError: undefined is not an object (evaluating 'key.substr')
    (anonymous function) (desk.min.js:119:165)
    dispatch (jquery.min.js:3:7543)
    dispatch (report.min.js:206)
    handle (jquery.min.js:3:5626)

function(r){me.parent.reportview.set_columns_and_filters(JSON.parse(frappe.get_doc("Report",me.docname).json)); ...

On the Page, there is a row with a filter: Document Status Not Equals Cancelled.

Any help appreciated. J

(on a free plan)