when you encounter the following problems in the process of learning vue, I hope you can help me.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>bus</title>
</head>
<body>
<div id="app">
<div>{{coun}}</div>
<addbtn :count="coun"></addbtn>
<subbtn :count="coun"></subbtn>
</div>
<script src="./vue.js"></script>
<script>
var busVM = new Vue();
var vm = new Vue({
el:"-sharpapp",
data:{
coun:0
},
components:{
"addbtn":{
template:`<button @click="addcount">+</button>`,
props:["count"],
methods:{
addcount(){
busVM.$emit("changeCount",PPthis.count);
}
}
},
"subbtn":{
template:`<button @click="subcount">-</button>`,
props:["count"],
methods:{
subcount(){
busVM.$emit("changeCount",this.count-1);
}
}
}
},
mounted:function(){
busVM.$on("changeCount",function(val){
this.coun = val
}.bind(this))
}
});
</script>
</body>
</html> 
an error like this will be reported when modified with PP in a subcomponent, but not with + 1. Why?