How to correctly pass a Heat template in Mistral heat.stacks_create action?

I am trying to deploy a Heat stack from a Mistral workflow using the built-in heat.stacks_create action. Mistral and Heat services are working fine, but whenever I provide a Heat template through the workflow, the execution fails with: “Template not in valid format: The template is not a JSON object or YAML mapping” The same Heat template deploys successfully with the standard openstack stack create command, so the template itself is valid. My question is: What is the correct and supported format for passing a Heat template into heat.stacks_create؟