Error in F4 help/popup

I tried a popup help as shown below...

<htmlb:inputField id = "empid"

showHelp = "true"

value = "<%= empid %>"

onValueHelp = "window.open('emppop.do','Emp Details','width=400,height=500')" />

But when I tried to click for that nothing is displayed but in the page status bar it is showing 'Error on page'. It seems like a javascript error...How can resolve it??