Ruby connecting to external Mongo DB - ruby-on-rails

Question

We have a main portal site (running Rails 4 with a PostgreSQL database), and an external image server (running Node.js with Mongo database). I'm trying to establish a connection from Rails to the database - I installed the mongo gem - https://docs.mongodb.org/ecosystem/drivers/ruby/ - and have been following the guide, but got stuck on an odd key error that I can't seem to find any information on.
The image server itself is running fine with no problems (it has a GUI interface that is working fine).
In my controller (I changed the names and left out the passwords):
image_server = Mongo::Client.new([ 'image.companyname.com:####' ], :database => 'db-name')
It seems to connect:
D, [2015-11-11T00:41:22.730360 #9410] DEBUG -- : MONGODB | Adding image.companyname.com:#### to the cluster.
But then just spams this message over and over (and queries don't do anything but return this error even faster).
D, [2015-11-11T00:41:22.991386 #9410] DEBUG -- : MONGODB | key not found: "t"
Eventually it returns one error message as well, but keeps spamming the key not found error as well:
Mongo::Error::NoServerAvailable (No server is available matching preference: #<Mongo::ServerSelector::Primary:0x007f5a943f6ee8 #options={"mode"=>:primary, "database"=>"db-name"}, #tag_sets=[], #server_selection_timeout=30>):
app/controllers/admin/model_controller.rb:9:in `index'
EDIT
I even tried connect directly to the UNIX socket, and got the same error:
image_server = Mongo::Client.new('mongodb://image.companyname.com:####/path/to/socket/socketname.sock')
END EDIT
I'm unsure what the heck this 'key not found "t"' error is, or how to even begin to diagnose this. I've messed with every single connection option I can think of, and nothing changes. Any ideas?