Baby Zombies cannot be summoned

When I do /summon Zombie ~ ~ ~ {IsBaby:1} in either chat or in a command block, a baby zombie is NOT summoned, even though the command is right. The error message reads: "Expected whitespace to end one argument, but found trailing data at position 7:simmon <--[HERE]"

I can, however, still obtain baby zombies by doing /summon chicken ~ ~ ~ {Passengers:[{id:zombie,IsBaby:1}]}