AppleScript & Numbers error - The file format is invalid.

Here is a simple AppleScript to open a file:

set FileName to choose file of type "numbers" with prompt "Open File"

tell application "Numbers"

set DocName to open FileName

activate

end tell

It works.

But I don't need the interaction, I'd prefer to just set FileName to a fixed string, like this:

set FileName to "Source.numbers"

tell application "Numbers"

set DocName to open FileName

activate

end tell

But at runtime I get this error:

Seems I need to apply (of type "numbers") to FileName somehow but I can't figure it out.

Any ideas?