Argument Types Do Not Match

   I am getting an error:

private async Task<List<WorkspaceEntity>> GetWorkspaces(String key) {

            var provider = new EntityManagerProvider<ShellEntityManager>();

            var uow = new UnitOfWork<WorkspaceEntity>(provider);

            var query = await uow.Entities.FindAsync(a=> a.ToolbarKey == key);

            return new List<WorkspaceEntity>(query);

        }

If I hard code a value for the "key" then it works fine. (It's just a simple string.)

This is the error that the BOS writes to the log:

Error System.ArgumentException: Argument types do not match at System.Linq.Expressions.Expression.Constant(Object value, Type type) at ....

This makes no sense to me.  This project is using 7.1.0