Updating JournalEntries object with more than one currency fails

I'm using SBO 6.50 (exactly CEE version 7.60.014 SP:01 EF:08).

I'm trying to update memo field of existing journal entries. Most of them works OK, but if I try to update entry with more than one currency, update fails with an error code -5002 and message:

Transaction includes more than one currency [OCRN].

Code (in C++) is quite simple:

SAPbobsCOM::ICompanyPtr m_company = SAPbobsCOM::ICompanyPtr("SAPbobsCOM.Company");
SAPbobsCOM::IJournalEntriesPtr pEntry = m_company->GetBusinessObject(SAPbobsCOM::oJournalEntries);
long m_nId = 1;	// TransId field, any valid value

try
{
	if (pEntry->GetByKey(m_nId) != -1)
		return;

	pEntry->Memo = _bstr_t("Memo");;
	if (pEntry->Update() < S_OK)
		throw(0);
}
catch(...)
{
	long lError; BSTR bstrError;
	m_company->GetLastError(&lError, &bstrError);
}

What's wrong? Using multiple currencies in journal entry is correct, it is possible to add them manually so it should be possible to update such an "safe" field as Memo is.