Closure Compiler warning for FileReader and Blob - cannot fix it

I run the following through Closure Compiler with advanced optimizations:

/** @param{Blob} a_blob */
function test(a_blob){
 /**@type{FileReader} */var filereader = new FileReader();
 filereader.onload = function(e){};
 filereader.readAsArrayBuffer(a_blob);
}

and I get:

WARNING - actual parameter 1 of FileReader.prototype.readAsArrayBuffer does not match formal parameter
found   : (Blob|null)
required: Blob
     filereader.readAsArrayBuffer(a_blob);
                              ^

What can I do to fix this? Is this a bug in CC?