Invalid default for field warning message coming from Avro?

I have a avro schema like this in which I have `is_userid_present` decalred
as boolean and I wanted to give default value as false to it so I came up
with below avro schema.

"type":"record",
"name":"hello",
"fields":[

"name":"u",
"type":[
"string",
"null"

},

"name":"p",
"type":"string"
},

"name":"bu",
"type":"boolean"
},

"name":"is_userid_present",
"type":"boolean",
"default":"false"

But from Avro code, I am seeing this warning message -

[WARNING] Avro: Invalid default for field is_userid_present: "false"
not a "boolean"

I am not sure what wrong I am doing here? If I have avro schema like this,
then will it be of any problem for writing and reading the data?