script5007: unable to get property 'join' of undefined or null reference in IE11

I see the error below on IE 11:

script5007: unable to get property 'join' of undefined or null reference

My function in JS is:

function BAOStringBuffer() { 
this.buffer = []; 
} 

BAOStringBuffer.prototype.append = function append(string) { 
this.buffer.push(string); 
return this; 
}; 

BAOStringBuffer.prototype.toString = function toString() { 
 return this.buffer.join(""); 
};

I don't see that error in IE8. Do you see that error? Can we fix it?

Regards,

Bao