Creating a HTML table with one intial row (e.g. with element "Row2") and then prepending rows (e.g. with elements "Row 1", "Row 0", "Row -1") with gnc:html-table-prepend-row! will end up in (("Row 2") "Row 1" "Row 0" "Row -1") as the table data list. For (gnc:html-table-get-cell test-table 2 0) this leads to an error: (wrong-type-arg "length" "Wrong type argument in position ~A: ~S" (1 "Row 1") ("Row 1"))
A solution proposal is covered in PR#391 see https://github.com/Gnucash/gnucash/pull/391/commits/c2f9197edbdc1940ae8666e3b38cdbbf29db1edc
Which is merged and will be released in GnuCash 3.5.