Subquery returns more than 1 row

Hi Guys!

When generating a sales by client report. i get this error:

Subquery returns more than 1 row

This only happens when there is a same record (like client). So when we sold twice a product to the same client in 1 week. and i want to generate report for that week, than i get this error:

So what we need is; a code that multi records are accepted in reports. Can someone HELP Please?

A PHP Error was encountered

Severity: Warning

Message: mysqli::query(): (21000/1242): Subquery returns more than 1 row

Filename: mysqli/mysqli_driver.php

Line Number: 307

Backtrace:

File: /home/medisch/domains/SNIP!/application/modules/reports/models/Mdl_reports.php
Line: 146
Function: get

File: /home/medisch/domains/SNIP!/application/modules/reports/controllers/Reports.php
Line: 32
Function: sales_by_client

File: /home/medisch/domains/SNIP!/index.php
Line: 325
Function: require_once

A Database Error Occurred

Error Number: 1242

Subquery returns more than 1 row