Namespace prefix dc on creator is not defined on line 111. Error code: 201

Namespace prefix dc on creator is not defined on line 11. Error code: 201
Namespace prefix dc on creator is not defined on line 111. Error code: 201
Namespace prefix dc on creator is not defined on line 216. Error code: 201
Namespace prefix dc on creator is not defined on line 260. Error code: 201
Namespace prefix dc on creator is not defined on line 302. Error code: 201

I get lots of errors, trying to import nodes from RSS feed. But I don't even use the dc:created field. How can I fix it?