state.cartitems.push is not a function at Store.setusercart

Am using Vue Js and am trying to add products to cart using Localstorage so that latter I can check them out. My code looks okay but am geting that error. Below is the code

			state : {
   							 cartitems: [],
			},

			getters : {
    						cartitems:(state )=> state.cartitems,
			},

			mutations : { 
    					setusercart :(state,usercartitem ) => (state.cartitems.push(usercartitem)), 
			 },

			actions : {
							async addstocktocart( { commit }, data) {
        						commit('seterror', '')
        						commit('setmsg'data. Titlele + 'has been added to Cart')
        						commit('setloader', 'addstocktocart')
        						commit('setusercart', data)
        						console.log(this.state.auth.usercartitems)	
    						},
				}