Boo.Lang.Compiler.CompilerError appears when using eval

For my game, I need to execute a code that’s generated by a php script. This is how I do it:

var www : WWW = new WWW (urlw);
yield www;

The generated code works perfectly because when I copy it from the browser and paste it in the script, it works. When I try with eval, this appears.

CompilationErrorsException: BCE0011: Boo.Lang.Compiler.CompilerError: An error occurred during the execution of the step ‘Boo.Lang.Compiler.Steps.EmitAssembly’: ‘Boolean’. —> System.ArgumentException: Boolean
at Boo.Lang.Compiler.Steps.EmitAssembly.NumericPromotionOpcodeFor (TypeCode typeCode, Boolean checked) [0x00000] in :0

Some guesses on why this might be happening?

I figured it out. Yield was causing the problem. All I had to change was:
var startCode:Function = null;
StartCoroutine(startCode() as IEnumerator);

and inside the code, add:
startCode = function () {