"Successfully copied." message when copy fails due to FOREIGN KEY constraint

I am trying to work out why PostgreSQL responds with "Successfully copied."
when I think the copy has failed.

I am testing a table with a FOREIGN KEY constraint which is working (yay!).
Although invalid data is not loading, PostgreSQL responds with a message
that looks as though it was successful.

test=> \copy assays from assay_test3.txt;
Successfully copied.
test=>

The number of rows in the table remains the same as before the copy.
"assay_test3.txt" contains a single row with data not meeting the foreign
key requirement.

Is there perhaps an "errorlevel" switch that needs to be turned up a bit?

I will post the table structure and load file if necessary.

Thanks
Colin Rothnie