Lightning button event.getSource method error

I am using Lightning button and on the click of the button I am calling a method in JS controller. I followed the below article to identify the button name but unsuccessful. The moment I start typing in value on the field it pops up the error.

Component bundle version : 40.0

https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/js_cb_which_button_pressed.htm

<div aura:id="lookup" class="slds-show">    
    <input class="slds-input" type="text" aria-haspopup="true" aria-autocomplete="list" role="combobox" aria-expanded="true" aria-activedescendant="" aria-owns="companyList" value="{!v.searchString}" updateOn="keyup" onkeyup="{!c.runSearch}" placeholder="Search Companies..."/>
    <p>You clicked: {!v.whichButton}</p>
    <lightning:button  aura:id="lookupbutton" class="slds-button slds-input__icon slds-text-color--default"  iconName="utility:search" variant="base" onclick="{!c.runSearch}" value="{!v.searchString}" />
</div>

Here is my JS controller:

runSearch : function(component, event, helper) {
//  if(event.keyCode == 13 || event.which == 13){ //Enter key
        var whichOne = event.getSource().getLocalId();
        console.log(whichOne);
        component.set("v.whichButton", whichOne);
        var searchString = event.target.value;
        component.set("v.searchString", searchString);
        component.set("v.SECompanies", null);
        console.log(searchString);
        helper.runSearchHelper(component);
//  }
}