Can't write to slave

I have a slave with slave-read-only set to no but when I attempt to do a SADD to it I got a No connection is available to service this operation.

at StackExchange.Redis.ConnectionMultiplexer.ExecuteSyncImpl[T](Message message, ResultProcessor`1 processor, ServerEndPoint server) in c:\TeamCity\buildAgent\work\3ae0647004edff78\StackExchange.Redis\StackExchange\Redis\ConnectionMultiplexer.cs:line 1857
at StackExchange.Redis.RedisBase.ExecuteSync[T](Message message, ResultProcessor`1 processor, ServerEndPoint server) in c:\TeamCity\buildAgent\work\3ae0647004edff78\StackExchange.Redis\StackExchange\Redis\RedisBase.cs:line 80
at StackExchange.Redis.RedisDatabase.SetAdd(RedisKey key, RedisValue[] values, CommandFlags flags) in c:\TeamCity\buildAgent\work\3ae0647004edff78\StackExchange.Redis\StackExchange\Redis\RedisDatabase.cs:line 921

When I issue SMEMBERS it works fine, I'm assuming it's a write problem.

Inspecting the variables while debugging, I noticed that the Server object has SlaveReadOnly set to false as expected, but there's AllowSlaveWrites also set to false. I don't know how those are set (or what exactly they affect), but I forcefully changed AllowSlaveWrites to true with no luck.

Setting abortConnect=false on configuration string didn't help either.

If I set the instance to master everything works fine.
It's reproducible in my development machine where the redis configuration is pretty much vanilla.
This is not an intermittent error, I get it as soon I start my application.

Start with the SADD and SMEMBERS entry points in StackExchange/Redis/RedisDatabase.cs, then trace ConnectionMultiplexer.ExecuteSyncImpl in ConnectionMultiplexer.cs around line 1857. Reproduce the commands against a Redis slave and inspect how SlaveReadOnly and AllowSlaveWrites affect server selection. Done means the cause of the write failure is identified and SADD behavior is corrected or clearly documented with coverage.