Foreign Key Question

Example:

Table 1:
Inventory, Productnr, Productserial
(PK Inventory, Prouctnr)

Table 2:
Pruductnr, Produktdesc, Productversion
(PK Productnr; FK Productnr)

This Construct gives me an error during the creation in Postgresql
(saying there is no unique constraint matching given keys for referenced
table xxxx). The Primary Key in table 1 consists of two columns. In the
second table there is a foreign key refencing to the first table.

A unique constraint doesn't make sense on table one. Am I missing
something here?
Thanks for any answer
Tuan