Just gone to 5.1 (very nice) and implemented the event module. Added an event. When in calendar display mode the word 'event' appears in parentheses at the top of the day where the event is. When I click this I get the following fatal error

Fatal error: Cannot use string offset as an array in .../includes/theme.inc on line 552

I'm using the Garland theme (Belgian chocolate colour scheme - lovely).

Now this doesn't particularly upset me but it does freak out the users. Any ideas as to what's going on here?