With secondary index, EXPLAIN and PREPARE fail when using placeholders

When there is a relevant secondary index and one tries to query it with placeholders (positional parameters or named parameters), the request fails due to a JSON marshalling issue.

To see it on beer-sample in cbq, first create an appropriate index:
CREATE INDEX cities ON `beer-sample` (city);

A few queries that work (the one with placeholder just complains that no value was passed for $1):
SELECT * FROM `beer-sample` WHERE city = 'Wrzburg';
SELECT * FROM `beer-sample` WHERE city = $1;
EXPLAIN SELECT * FROM `beer-sample` WHERE city = 'Wrzburg';

But explain and prepare with the placeholder fail:
EXPLAIN SELECT * FROM `beer-sample` WHERE city = $1;
PREPARE SELECT * FROM `beer-sample` WHERE city = $city;

The response is:
cbq> EXPLAIN SELECT * FROM `beer-sample` WHERE city = $city;
{
"requestID": "716c77ff-8120-4433-82bc-2dc10691b34e",
"signature": "json",
"results": [
],
"errors": [

{ "code": 5000, "msg": "Failed to marshal JSON. - cause: json: error calling MarshalJSON for type *plan.Sequence: json: error calling MarshalJSON for type plan.Operator: json: error calling MarshalJSON for type expression.Expression: invalid character '$' looking for beginning of value" }

],
"status": "stopped",
"metrics":

{ "elapsedTime": "685.526us", "executionTime": "606.07us", "resultCount": 0, "resultSize": 0, "errorCount": 1 }

}