Fun with ArrayBuffers

I'm hoping somebody can tell me where I am going wrong using typed array buffers. I am parsing a binary stream. Uint32 drop out very nicely using this function:

  readUint32: function(stream) {
     var u = new Uint8Array(stream.readByteArray(4));
     return new Uint32Array(u.buffer)[0];
   },

But the equivalent for Float32 fails. I just cannot work out why....

  readFloat32: function(stream) {
     var u = new Uint8Array(stream.readByteArray(4));
     return new FLOAT32Array(u.buffer)[0];
   },

Any help very gratefully received.