Loop through linked list in kernel space

I'm trying to loop over vma's (referenced by the pointer mm) inside of the task_struct. What I'm doing is trying to print some of the attributes of the struct vm_area_struct (specifically, I'm trying to print vm_start). My issue is that I keep getting a NULL pointer dereference error.

void print_mmap(struct vm_area_struct *mmap){    while(mmap->vm_next != NULL)    {          printk(KERN_INFO "vm_start: %lu\n", mmap->vm_start);          mmap = mmap->vm_next;     }}

The error I get is

[  177.348231] BUG: kernel NULL pointer dereference, address: 0000000000000010[  177.348235] #PF: supervisor read access in kernel mode[  177.348237] #PF: error_code(0x0000) - not-present page

Can anyone help me loop until I reach the end of this linked list without throwing an error?