Steps to reproduce issue

1. Prepare a CSV file with two lines:
name
Uploaded cohort 1

2. Go to Site administration > Users > Cohorts > Upload cohorts.
3. Click Choose a file..., select the above CSV file and click Upload this file.
4. Click Preview then click Upload cohorts.
When the message Uploaded 1 cohorts is displayed click Continue.

Expected results

The new cohort appears in the list of cohorts.

Actual results

Exception - core_cohort\reportbuilder\local\systemreports\cohorts::core_cohort\reportbuilder\local\systemreports{closure}(): Argument #1 ($idnumber) must be of type string, null given, called in [dirroot]/reportbuilder/classes/local/report/column.php on line 690

Debug info:
Error code: generalexceptionmessage

Stack trace:

line 142 of /cohort/classes/reportbuilder/local/systemreports/cohorts.php: TypeError thrown
line 690 of /reportbuilder/classes/local/report/column.php: call to core_cohort\reportbuilder\local\systemreports\cohorts::core_cohort\reportbuilder\local\systemreports{closure}()
line 205 of /reportbuilder/classes/table/system_report_table.php: call to core_reportbuilder\local\report\column->format_value()
line ? of unknownfile: call to core_reportbuilder\table\system_report_table::core_reportbuilder\table{closure}()
line 207 of /reportbuilder/classes/table/system_report_table.php: call to array_walk()
line 2016 of /lib/tablelib.php: call to core_reportbuilder\table\system_report_table->format_row()
line 2140 of /lib/tablelib.php: call to table_sql->build_table()
line 58 of /reportbuilder/classes/output/renderer.php: call to table_sql->out()
line 491 of /lib/outputrenderers.php: call to core_reportbuilder\output\renderer->render_system_report_table()
line 113 of /reportbuilder/classes/external/system_report_exporter.php: call to plugin_renderer_base->render()
line 125 of /lib/classes/external/exporter.php: call to core_reportbuilder\external\system_report_exporter->get_other_values()
line 72 of /reportbuilder/classes/output/system_report.php: call to core\external\exporter->export()
line 45 of /reportbuilder/classes/output/renderer.php: call to core_reportbuilder\output\system_report->export_for_template()
line 491 of /lib/outputrenderers.php: call to core_reportbuilder\output\renderer->render_system_report()
line 231 of /reportbuilder/classes/system_report.php: call to plugin_renderer_base->render()
line 118 of /cohort/index.php: call to core_reportbuilder\system_report->output()