VideoPlayer and 'source'?

[Richmond]

Mucking around with the VideoPlayer (something I haven't done for
about 7 years) I wondered how one sets the SOURCE. When I typed

on mouseUp
   set the source of player "MyPlayer" to "abc/def/ghi.mov"
end mouseUp

while the script editor stated that that was fine, I still ended up with
the jolly blue of "Sample.mov"

any ideas?