ORA-20003: Failed ORA-06531: Reference to uninitialized collection while cancelling Oracle Purchase

We are trying to cancel a purchase order using this API call from a concurrent program:

   BEGIN

PO_Document_Control_PUB.control_document (

p_api_version => 1.0,

p_init_msg_list => FND_API.G_TRUE,

p_commit => FND_API.G_FALSE,

x_return_status => l_cancel_return_status,

p_doc_type => 'PO',

p_doc_subtype => 'STANDARD',

p_doc_id => null,

p_doc_num => indx.phi_document_num,

p_release_id => null,

p_release_num => null,

p_doc_line_id => null,

p_doc_line_num => indx.pli_line_num,

p_doc_line_loc_id => null,

p_doc_shipment_num => null,

p_action => 'CANCEL',