Setting initial value in state when subgraph is invoked

[sgsharma]

When I invoke a subgraph with START like so from a parent graph:

subgraph_builder.add_node("node_1", subgraph_node_1)
subgraph_builder.add_node("node_2", subgraph_node_2)
subgraph_builder.add_edge(START, "node_1")
subgraph_builder.add_edge("node_1", "node_2")

I receive an InvalidUpdateError("Must write to at least one of...") for a field that is set in the parent graph. Is this expected? Is there a way to bypass this?