I've seen but used like that before, but it's a bit archaic.
if you throw the word \"that\" in there, it makes more sense if you're not used to the word used that way: \"There is no mother but that loves her children.\" The word \"that\" is not a strict requirement, however.
http://dictionary.reference.com/browse/but
It shouldn't be in an ESL text, though, because it's a very rare usage.