Preparing UPDATE queries with collections returns suboptimal metadata

CQL3, binary protocol.

collections (id int primary key, amap map<int, varchar>);

preparing "UPDATE test.collections SET amap[?] = ? WHERE id = ?" returns the following metadata:
[{column,<<"test">>,<<"collections">>,<<"amap">>,
{map,int,varchar}},
{column,<<"test">>,<<"collections">>,<<"amap">>,
{map,int,varchar}},

]

Ideally it should return [int, varchar, int] types. Less ideally [

, int] and expect an encoded map with a single key-value pair. But certainly not what it currently returns.