[CONNECT C++] Adding an entry into the StringList of a ComboBox

I've got ComboBox with three entries from a StringList.

Now I wish to programmatically add an entry:

comboBoxDiP = mdlDialog_itemGetByIndex(GetDialog(), 7);
		if (comboBoxDiP)
		{
			wprintf(L"Got it\n");
			// test to insert a value
			StringListP		strListP = mdlDialog_comboBoxGetStrListP(comboBoxDiP->GetRawItem());
			if (strListP)
			{	// add an item into the string list
				long			newIndex;
				mdlStringList_insertMember(&newIndex, strListP, -1, 1);
				wprintf(L"newIndex=%d\n", newIndex);
				long			result = mdlStringList_setMember(strListP, newIndex, L"999.0", NULL);
				wprintf(L"result=%d\n", result);
				ErrorCode		ec = mdlDialog_comboBoxSetStrListP(comboBoxDiP->GetRawItem(), strListP, 1);
				wprintf(L"ec=%d\n", ec);
			}
		}
		else

Once the code runs, this is what the ComboBox look like when opened:

This is the "output" of the print statements, showing everything went OK...

Any idea where I've gone astray?

Bruce