Webform date field: Fatal error: Cannot create references to/from string offsets

When submitting a webform with a date field I'm getting a fatal error:

"Fatal error: Cannot create references to/from string offsets"

It's being called by "drupal_array_set_nested_value( )" for the date field. It looks like the form_state input value for that field is a string (the year) which seems to be the root of the issue because then it is trying to add a month and a day to it as if it were an array.