I'm trying to understand how custom universes work.  The following backtest should buy the four symbols specified, liquidate a week later and buy the four new symbols.  Instead it is throwing an ambiguous error message after making the first set of purchases but before reading the next universe.  The error message is:  "Backtest Handled Error:   not found in portfolio. Request this data when initializing the algorithm."  What am I doing wrong?

For extra credit: I'm working toward an MACD crossover system using Minute data consolidated to 30 minute bars applied to a dynamic list of symbols.  If there is an example of such it would be nice.  Thanks for any help.