"Invalid value for field(s) : field ID;" when attempting to import activities with custom data fields

I came across the same problem as described here: http://forum.civicrm.org/index.php?topic=20742.0 but I couldn't find a bug report for it.

I was able to replicate this on d7.demo.civicrm.org running build 36786. I also replicated this on a fresh 4.0.7 install.

Steps:

• Import contact records.
• Create a new Activity Type.
• Create a Custom Data Set for the activity type.
• Create one or more Custom Data fields in the Custom Data Set.
• Attempt to import activities, mapping one of the fields to be imported to the custom data field.

All records imported are rejected with: "Invalid value for field(s) : field ID;"

The error disappears if you remove the mapping to the custom field.