Bad Request: Invalid null value for clustering key part

[Petter von Dolwitz (Hem)]

I'm trying to import some rows to a table using the COPY command. I have a
PRIMARY KEY that is built up from many fields. During the import I get

Bad Request: Invalid null value for clustering key part {field_name}

Is it not permitted to have null values in a field that is part a primary
key? This seems to be ok when using the java-driver to insert row-by-row
but not when using the COPY command. Can anybody advise?

Thanks,
Petter