shell: HERE document - mark of EOF does not work in function

Question

Having this simple script:

#!/bin/bash
function func () {
   cat <<- EOF
   some stuff
   EOF
   return
}

Gives

 ./a: line 7: warning: here-document at line 4 delimited by end-of-file
 (wanted `EOF') ./a: line 9: syntax error: unexpected end of file

What's wrong with my HERE mark?