He remained sitting on the floor and held his breath , wondering how she knew that he had come back .<br>If she said ` Peter ' again , he meant to cry ` Mother ' and run to her .<br>But she spoke no more , she made little moans only , and when he next peeped at her she was once more asleep , with tears on her face .<br>It made Peter very miserable , and what do you think was the first thing he did ?<br>Sitting on the rail at the foot of the bed , he played a beautiful lullaby to his mother on his pipe .
Peter sang his mother a song. 
The statement was incorrect, I'm not sure why the system didn't get it.  He played a lullaby on a pipe for his mother, he did not sing her a song. 
( z1 sing :ARG0 ( z2 person :name ( z3 name :op "Peter" ) ) :ARG1 ( z4 song ) :ARG2 ( z5 person :ARG0-of ( z6 have-rel-role :ARG1 z2 :ARG2 ( z7 mother ) ) ) )