packer error : setting pillar in salt_call_args for salt-masterless

[spear]

I am getting packer validation error for curly brackets when set in salt_call_args arguments value.

"salt_call_args": "pillar="\\{'test_mode': True\\}" for salt-masterless provisioner.

Error : Failed to parse template: Error parsing JSON: invalid character '\\' after object key:value pair

Also tried : "salt_call_args": "pillar={'test_mode': True\\}" 

Is there any example available for escaping the curly bracket characters {, }.  highly appreciate your response on this.