RuntimeError: std::exception in 'SimTK::State & OpenSim::Model::initSystem()'

[hosseinahm1367]

Hi, (Version: 4.1, System: Mac)

I tried running the "dynamic_walker_example_optimizaion" python script. I ended up with this error:

RuntimeError: std::exception in 'SimTK::State & OpenSim::Model::initSystem()': Failed to connect Socket 'child_frame' of type PhysicalFrame (details: Model'model'::findComponent() cannot find a nameless subcomponent.). In Object 'PlatformToGround' of type WeldJoint. Thrown at Component.cpp:287 in finalizeConnections().

How can I go about fixing this issue?