Adding custom activity to user

CiviCRM Build: March 13 (sorry I don't have the build number, let me know if it is stored in a file somewhere)
Drupal: 4.7 cvs

Task: Add activity to a contact record
Expected Behavior: Fill out contact form, submit, activity is attached to record.
Observed Behavior: Fill out form, submit, error message returned:

------------

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -3
[message] => DB Error: constraint violation
[mode] => 16
[debug_info] => INSERT INTO civicrm_activity (source_contact_id , activity_type_id , target_entity_table , target_entity_id , subject , scheduled_date_time , duration_hours , duration_minutes , location , details , status ) VALUES ( 0 , 6 , 'civicrm_contact' , 4 , 'This is an example of mark\'s activity type' , 20060322150000 , 0 , 0 , 'here' , NULL , 'Scheduled' ) [nativecode=1216 ** Cannot add or update a child row: a foreign key constraint fails]
[type] => db_error
[user_info] => INSERT INTO civicrm_activity (source_contact_id , activity_type_id , target_entity_table , target_entity_id , subject , scheduled_date_time , duration_hours , duration_minutes , location , details , status ) VALUES ( 0 , 6 , 'civicrm_contact' , 4 , 'This is an example of mark\'s activity type' , 20060322150000 , 0 , 0 , 'here' , NULL , 'Scheduled' ) [nativecode=1216 ** Cannot add or update a child row: a foreign key constraint fails]
[to_string] => [db_error: message="DB Error: constraint violation" code=-3 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_activity (source_contact_id , activity_type_id , target_entity_table , target_entity_id , subject , scheduled_date_time , duration_hours , duration_minutes , location , details , status ) VALUES ( 0 , 6 , 'civicrm_contact' , 4 , 'This is an example of mark\'s activity type' , 20060322150000 , 0 , 0 , 'here' , NULL , 'Scheduled' ) [nativecode=1216 ** Cannot add or update a child row: a foreign key constraint fails]"]
)

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Errors are thrown for both custom and standard activity types.

I am unsure what foreign key this refers to. I will happily supply details from my mySQL tables if this would help.

The database contains 20 test records, all successfully uploaded.