Error if Date Field Null for content type that is Signup-enabled

I have a content type Course that has an optional start/end date/time, and the content type is enabled for signup, but is also optional.

I had no problems with previous releases, but in the latest update (6.x-1.0-rc6) I get the following error for a new Course node with no date/time values set (ie, using Blank default values), regardless of whether signup is enabled/disabled for this node:

warning: date_timezone_set() expects parameter 1 to be DateTime, null given in .../sites/all/modules/signup/includes/date.inc on line 485.
warning: date_modify() expects parameter 1 to be DateTime, null given in .../sites/all/modules/signup/includes/date.inc on line 488.
warning: date_format() expects parameter 1 to be DateTime, null given in .../sites/all/modules/signup/includes/date.inc on line 489.

If a start date/time is entered, the error does not appear. This error only appears on initial create; subsequent edit/save operations do not generate this error. Time zone handling is set to Site's time zone.

Any thoughts?