fromFactor

I have a login button and i had written tss for it

"#loginbutton[platform=ios formFactor=handheld]":{
 
    top:"49%",
    width:"90%",
    backgroundColor:'#2365C2',
    color:"#fff",
    borderRadius:5,
    font:{
        fontSize: "13sp",
        fontWeight:"bold"
    },
    height:"6%",
    style:Ti.UI.iPhone.SystemButtonStyle.PLAIN
},
"#loginbutton[platform=ios formFactor=tablet]":{
 
    top:"49%",
    width:"90%",
    backgroundColor:'#2365C2',
    color:"#fff",
    borderRadius:5,
    font:{
        fontSize: "25sp",
        fontWeight:"bold"
    },
    height:"6%",
    style:Ti.UI.iPhone.SystemButtonStyle.PLAIN
}

but i'm getting error as

undefined is not an object(evaluating Alloy.isHandHeld) and for ipad undefined is not an object(evaluating Alloy.isTablet)

kindly suggest me how to use form factor for handheld device and tablet.I have problem with my fonts so using form Factor