Don't worry if ptr is NULL, this check should have been made one level above.

src/linked_lists/linked_lists.c

@@ -29,9 +29,7 @@ void * get_next(void *ptr) {
 #ifdef BLOCKS_IN_DLL
 
 void set_previous(void *ptr, void *previous_block) {
-    if( ptr != NULL) {
-        get_header(ptr)->pointers.previous = (list_node_ptr) previous_block;
-    }
+    get_header(ptr)->pointers.previous = (list_node_ptr) previous_block;
 }
 
 void * get_previous(void *ptr) {
@@ -74,7 +72,7 @@ void remove_block(void **block, void **starting_node) {
         previous_node = current_node;
     }
 }
-#endif
+#endif /* BLOCKS_IN_SLL */
 
 #ifdef BLOCKS_IN_DLL
 void remove_block(void **block, void **starting_node) {