The error is coming from the calljs plugin, hence cjs_plugin.js.
The ReadExecutionReturn expression is not returning a string.
I can replicate the error with two actions:
CallJS| execute "0"
Text | set text to CallJS.ReadExecutionReturn
It's not a fatal since "0" is returned just fine. The assertion failure is also triggered when calling ReadExecutionReturn if js code with syntax errors is run.
To eliminate the assert edit \CallJS\runtime.js and find ReadExecutionReturn which should look like this:
exps.ReadExecutionReturn = function (ret)
{
ret.set_string(this.returnValue);
};
exps.ReadExecutionReturn = function (ret)
{
ret.set_string(String(this.returnValue));
};