elasticsearch 7.4.1, mapping "source" field error

[tanxiaodan]

I try:
elasticsearch 7.4.1
PUT test_source_field_name
{

"mappings": {
    "properties": {
        "source": {
            "type": "text"
        }
    }

}

}

exception:
{
"error": {
"root_cause": [
{
"type": "mapper_parsing_exception",
"reason": "Expected map for property [fields] on field [source] but got a class java.lang.String"
}
],
"type": "mapper_parsing_exception",
"reason": "Failed to parse mapping [_doc]: Expected map for property [fields] on field [source] but got a class java.lang.String",
"caused_by": {
"type": "mapper_parsing_exception",
"reason": "Expected map for property [fields] on field [source] but got a class java.lang.String"
}
},
"status": 400
}

But in kibana 7.4.1 , It's no problem.