diff --git a/containers.h b/containers.h
index 06086e0..d5f6c26 100644
--- a/containers.h
+++ b/containers.h
@@ -416,7 +416,7 @@ typedef struct internal_deque *deque;
 deque deque_init(size_t data_size);
 
 /* Utility */
-int deque_size(deque me);
+size_t deque_size(deque me);
 int deque_is_empty(deque me);
 int deque_trim(deque me);
 void deque_copy_to_array(void *arr, deque me);
@@ -431,12 +431,12 @@ int deque_pop_back(void *data, deque me);
 
 /* Setting */
 int deque_set_first(deque me, void *data);
-int deque_set_at(deque me, int index, void *data);
+int deque_set_at(deque me, size_t index, void *data);
 int deque_set_last(deque me, void *data);
 
 /* Getting */
 int deque_get_first(void *data, deque me);
-int deque_get_at(void *data, deque me, int index);
+int deque_get_at(void *data, deque me, size_t index);
 int deque_get_last(void *data, deque me);
 
 /* Ending */