how to correctly define using scm_define ?

[Tano Fotang]

I am trying to define some variable (libguile 1.7.1)  but it's going the 
wrong way all day:

SCM tt;
void set_tt(SCM proc){ //called often to set tt
 assert(scm_is_true(scm_procedure_p((proc)))); //passes
 scm_define(tt, proc);
 assert(scm_is_true(scm_procedure_p((tt)))); //fails
}

So I decide to skip scm_define and just do:

   tt = proc;

That appears okay until  the following line is seen some minutes later:

   scm_apply_0(tt, args)

then this error is displayed:

wrong-type-arg: [apply] Wrong type argument in position 1: #<freed cell 
0x406303b0; GC missed a reference>

I'd be glad for any pointers on how to proceed.
/fotang