diff --git a/cv4/list_ops.c b/cv4/list_ops.c
index 89d2ada..cf459c4 100644
--- a/cv4/list_ops.c
+++ b/cv4/list_ops.c
@@ -7,6 +7,9 @@
 list_t *new_list(size_t length, list_element_t elements[]) {
     list_t *list = malloc(sizeof(list_t) + length * sizeof(list_element_t));
     list->length = length;
+    if (NULL == list) {
+        return NULL; 
+    }
     for (size_t i = 0; i < length; i++) {
         list->elements[i] = elements[i];
     }
@@ -15,6 +18,9 @@ list_t *new_list(size_t length, list_element_t elements[]) {
 
 list_t *append_list(list_t *list1, list_t *list2) {
     list_t *list3 = malloc(sizeof(list_t) + (list1->length + list2->length) * sizeof(list_element_t));
+     if (NULL == list3) {
+        return NULL; 
+    }
     memcpy(list3,list1,sizeof(list_t));
     for(size_t i = sizeof(list_t); i < (sizeof(list_t) + (list1->length + list2->length));i++){
       list3->elements[i] = list2->elements[i];
@@ -32,6 +38,9 @@ list_t *filter_list(list_t *list, bool (*predicate)(list_element_t)) {
    	}
    }
    list_t *result = malloc(list->length);
+    if (NULL == result) {
+        return NULL; 
+    }
    for(size_t i = 0;i < list->length;i++)
    		result->elements[i] = filter[i];
    return result;
@@ -43,6 +52,9 @@ size_t length_list(list_t *list) {
 
 list_t *map_list(list_t *list, list_element_t (*mapper)(list_element_t)) {
     list_t *result = malloc(list->length);
+     if (NULL == result) {
+        return NULL; 
+    }
 	for (size_t i = 0; i < list->length; i++) {
         result->elements[i] = mapper(list->elements[i]);
     }
@@ -54,7 +66,9 @@ list_t *map_list(list_t *list, list_element_t (*mapper)(list_element_t)) {
 
 list_element_t foldl_list(list_t *list, list_element_t initial, list_element_t (*folder)(list_element_t, list_element_t)) {
     list_element_t result = initial;
-    
+     if (NULL == result) {
+        return NULL; 
+    }
     for (size_t i = 0; i < list->length; i++) {
         result = folder(result, list->elements[i]);
     }
@@ -64,7 +78,9 @@ list_element_t foldl_list(list_t *list, list_element_t initial, list_element_t (
 
 list_element_t foldr_list(list_t *list, list_element_t initial, list_element_t (*folder)(list_element_t, list_element_t)) {
     list_element_t result = initial;
-    
+     if (NULL == result) {
+        return NULL; 
+    }
     for (size_t i = list->length; i > 0; i--) {
         result = folder(list->elements[i - 1], result);
     }
@@ -74,6 +90,9 @@ list_element_t foldr_list(list_t *list, list_element_t initial, list_element_t (
 
 list_t *reverse_list(list_t *list) {
     list_t *list2 = malloc(list->length); 
+     if (NULL == list2) {
+        return NULL; 
+    }
     for (size_t i = 0; i < list->length; i++) {
         list2->elements[i] = list->elements[list->length - i - 1];
     }