InvokeURL - Execution exception : 'Error due to - 'Internal Exception''

InvokeURL - Execution exception : 'Error due to - 'Internal Exception''

I want to be able to download an image/pdf from a url and attach it to a record in CRM.  My code is as follows:

file_attach = invokeurl
[
      url :imageurl
      type :GET
];

info file_attach;

file_attach.setFileName(file_attach_name);

new_attachment = zoho.crm.attachFile("People",personid, file_attach);


The variable imageurl is in the right format and if pasted into a web browser, the image is shown. 

The response I get is:

Failed to execute function
  • Execution exception : 'Error due to - 'Internal Exception''
Any idea what I'm doing wrong?