Invalid or unparsable filter request, PHP Server-Side Scripting

I am writing a PHP Server-Side script and upon running a get request on my exchange_rates table, I was able to retrieve all the records in that table.

However, the below error is returned whenever I attempt to apply a filter to the query, eg: $url = $db_service.’/exchange_rates?select=rate,id,inverse&filter=source_currency=GBP’;

Error Response:
“error”: {
“code”: 500,
“message”: “error code = 400, messsage = Invalid or unparsable filter request.”

I have been siting on this for almost 2 weeks and now in need of urgent help. I will be grateful for a quick response.