Yes, I plan on having default handling that would make it ignore the terrain: definition for normal maps or bad definitions.
Did the log print a message like "Map generator encountered an error: invalid terrain for adding block from alternate terrain." for the crash? I put in that error when the command is given the name of a terrain that doesn't exist, or doesn't load a terrain... I think I'll have it load the normal terrain when none is defined.
Edit: How did you use the command? Would you mind posting the ruleset you used to try it?
More edit: For moving the alternate terrain handling into addBlock/fillArea, I would have the code check if there's a valid definition of "terrain:" in the command, and load the map from that if there is. If there is no definition, or you put "terrain: default" in the ruleset, it pulls the map blocks from the same terrain as the rest of the map.