problem GetUserFace

[nativecode2020]

Hi, I have a ZKTeco Iface 1000 and need to use the GetUserFace function, but it's returning null.

ICollection templates = new List();
string faceData = "";
int faceLength = 0;

for (int faceIndex = 0; faceIndex < 10; faceIndex++)
{
if (objCZKEM.GetUserFaceStr(machineNumber, enrollNumber, faceIndex, ref faceData, ref faceLength))
{
templates.Add(new UserTemplate
{
TemplateIndex = faceIndex,
TemplateData = faceData
});
}
}