diff --git a/contracts/utils/EnumerableSet.sol b/contracts/utils/EnumerableSet.sol
index 8917cccaf17..e6f03161224 100644
--- a/contracts/utils/EnumerableSet.sol
+++ b/contracts/utils/EnumerableSet.sol
@@ -53,21 +53,10 @@ library EnumerableSet {
      */
     function _add(Set storage set, bytes32 value) private returns (bool) {
         if (!_contains(set, value)) {
-
-            uint256 last;
-            assembly {
-                let p := sload(0x40)
-                mstore(p, set.slot)
-                let blk := keccak256(p, 0x20)
-                // push new value
-                last := sload(set.slot)
-                sstore(add(blk, last), value)
-                // update array length
-                last := add(last, 1)
-                sstore(set.slot, last)
-            }
-            set._indexes[value] = last;
-
+            set._values.push(value);
+            // The value is stored at length-1, but we add 1 to all indexes
+            // and use 0 as a sentinel value
+            set._indexes[value] = set._values.length;
             return true;
         } else {
             return false;
@@ -132,7 +121,7 @@ library EnumerableSet {
     *
     * - `index` must be strictly less than {length}.
     */
-    function _at(Set storage set, uint256 index) private view returns (bytes32 value) {
+    function _at(Set storage set, uint256 index) private view returns (bytes32) {
         require(set._values.length > index, "EnumerableSet: index out of bounds");
         return set._values[index];
     }