Wrong Attribute size in HL image creation?

For example, when I create an image with `H5IMmake_image_8bit`, the CLASS attribute is created with size 6, although it should be 5 according to the specification.
Do I understand something wrong here?
So when I create attributes in C, should their length be strlen+1 as in the [HL library](https://github.com/HDFGroup/hdf5/blob/9bb1d6d78e305e73b5c882e8f5b3ef66a1acdf8d/hl/src/H5LT.c#L3559) or just strlen?

Thanks for the help