API Error with % in string

API Error with % in string

I had to remove % from a value when updating a multi-line field in account otherwise I got an error.I tried to escape it but that didn't work. The error is:
code:4600
message:Unable to process your request. Please verify if the name and value is appropriate for the "xmlData" parameter.
Is this an XML restriction or a Zoho restriction and how do I add a value with a % in it?

Thanks
Glenn