Error: 'Element " a[data-vars-ei] " is required to be an AMP element' on GA tracking of element

i am trying to track visibility of an element and its showing console error:'Element " a[data-vars-ei] " is required to be an AMP element'. But in case of click tracking similar thing is working fine. I am unable to understand why this is happeing and what I should do to send tracking. below is my code i have written to track events

"tracklinkClicks":{

                "on": "click",
                "selector": "a[data-vars-ec], div[data-vars-ec], span[data-vars-ec]",
                "request": "event",
                "vars":{
                "eventCategory": "${ec}",
                "eventAction": "${ea}",
                "eventLabel": "${el}"
                }
            },
            "trackimpressions":{
                "on": "visible",
                "selector": "a[data-vars-ei]",
                "request": "event",
                "vars":{
                "eventcategory": "${ec}",
                "eventaction": "${ei}",
                "eventlabel": "${el}"
                },
                "extraUrlParams":{
                "ni":"1"
                }
            },