Seg fault while implementing malloc, realloc, free in C - c

Question

I am implementing a simple version of malloc, realloc, and free for an assignment and having trouble debugging. My code seems to be working for malloc, but the realloc tests are resulting in a seg fault. Specifically, the pointer passed to free() seems to be the problem.
There is a "free list" to manage the list of previously allocated blocks of memory. Each node in this list maintains the next and previous blocks, and an int free which is set to 1 when the memory is available, 0 otherwise.
void *mm_malloc(size_t size) {
if (size <= 0) return NULL;
struct list_node *node;
if (!list_head) {
node = request_block(size);
list_head = node;
list_tail = node;
} else {
node = get_free_block(size);
if (!node) { //no available existing block
node = request_block(size);
if (!node) { //request failed
return NULL;
}
} else { //available existing block
//TODO: split block
node->free = 0;
}
}
return memset(node+1, 0, node->size);
}
void *mm_realloc(void *ptr, size_t size) {
if (size <= 0) return NULL;
if (!ptr) return mm_malloc(size);
struct list_node *node = (struct list_node*)ptr - 1;
if (node->size >= size) {
// TODO: free extra space
return ptr;
}
void *new_block = mm_malloc(size);
if (!new_block) return NULL;
memcpy(new_block, ptr, node->size);
free(ptr); //Error happens with this call.
return new_block;
}
void mm_free(void *ptr) {
if (!ptr) return;
struct list_node *node = (struct list_node*)ptr - 1;
node->free = 1;
}
EDIT: left out some important helper functions
struct list_node *get_free_block(size_t size) {
struct list_node *curr = list_head;
while (curr && !(curr->free && curr->size >= size)) {
curr = curr->next;
} return curr;
}
struct list_node *request_block(size_t size) {
struct list_node *node = sbrk(0);
void *request = sbrk(size + NODE_SIZE);
if (request == (void*) -1) { // attempted sbrk failed
return NULL;
}
if (list_tail) {
node->prev = list_tail;
list_tail->next = node;
list_tail = node;
}
node->next = NULL;
node->free = 0;
node->size = size;
return node;
}