Describe the bug
The attached SAM file throws an error because price signals dispatch has trouble interpreting the utility rate, with the absence of period 1 in month 5. However the utility rates code has no trouble with this rate.

To Reproduce
Steps to reproduce the behavior:

1. Run the attached SAM file
   Hundt.demandraterror.zip

2. See error

Expected behavior
Price signals dispatch should support all rates supported by the utility rates code.