blanks don't get saved in math-text mode

[hinten hoch]

Dear all,
I'm trying to use the qtree package in lyx to generate some trees in
math-text mode. In principle it's working fine, but when I save the document
or copy and paste the trees, the blanks that are necessary to get the right
results get deleted. Protected blanks do get saved, but don't work. Is there
a way to keep the blanks in there?
Thanks much,
Andi