Error jscript form OnLoad() event

Hi, I'm getting an error after adding a JS library, and function below is assigned to 
OnLoad event.  

function ShowCustomerFields()

{
	//    Xrm.Utility.alertDialog(" Test");
}

tested with an empty function, still get the error 

one of the scripts for this record has caused an error
typeError:ShowCustomerFields() is anot a function at eval(eval atRunHandleInternal ...

any ideas what causes this ? Thanks