Getting null error for content taxonomy dropdown when I save node

user warning: Column 'field_business_category_value' cannot be null query: INSERT INTO content_field_business_category (vid, nid, delta, field_business_category_value) VALUES (428, 123, 1, NULL) in /home/nash/public_html/sites/all/modules/contrib/cck/content.module on line 1213.

When I change the taxonomy of a multi-select content taxonomy dropdown it gives me this error. It looks like it saves the new terms right but it's trying to add the old terms as NULL values? Very strange...