c - 'Empty tree' function causing program to crash -

i have empty_tree() function returns true value if tree empty , returns false if not.

bool empty_tree(avl self) {     if (self == null)     {         return true;     }     else     {         return false;     } } 

it exists program easier understand without comments. now, works fine everywhere else in program. when try , use in insert function in place of working:

if(self == null) 

my program crash. isn't big deal , using 'if (self==null) suffice, wondering if had ideas why cause program crash. said, it's not extremely important won't bother including more code, if had guesses, i'd appreciate hearing thoughts.

edit: upon request, here's section works:

if (self == null)     {         self = (avl)malloc(sizeof(struct avlnode));         self->student_id = id;         self->left = self->right = null;     } 

and here's code will, when changed this, crash program:

if (!empty_tree(self))     {         self = (avl)malloc(sizeof(struct avlnode));         self->student_id = id;         self->left = self->right = null;     }