diff --git a/src/WakuRlnV2.sol b/src/WakuRlnV2.sol
index cc2127f..e1275b1 100644
--- a/src/WakuRlnV2.sol
+++ b/src/WakuRlnV2.sol
@@ -32,7 +32,7 @@ contract WakuRlnV2 {
     uint8 public constant DEPTH = 20;
 
     /// @notice The size of the merkle tree, i.e 2^depth
-    uint32 public SET_SIZE;
+    uint32 public immutable SET_SIZE;
 
     /// @notice The index of the next member to be registered
     uint32 public idCommitmentIndex = 0;
diff --git a/test/WakuRlnV2.t.sol b/test/WakuRlnV2.t.sol
index 756c472..e9102d5 100644
--- a/test/WakuRlnV2.t.sol
+++ b/test/WakuRlnV2.t.sol
@@ -109,13 +109,10 @@ contract WakuRlnV2Test is Test {
 
     function test__InvalidRegistration__FullTree() external {
         uint32 userMessageLimit = 2;
-        // we modify the set_size to be small so the test can run faster
-        stdstore.target(address(w)).sig("SET_SIZE()").checked_write(1);
-        vm.pauseGasMetering();
-        w.register(1, userMessageLimit);
-        vm.resumeGasMetering();
+        // we progress the tree to the last leaf
+        stdstore.target(address(w)).sig("idCommitmentIndex()").checked_write(1 << w.DEPTH());
         vm.expectRevert(FullTree.selector);
-        w.register(2, userMessageLimit);
+        w.register(1, userMessageLimit);
     }
 
     function test__InvalidPaginationQuery__StartIndexGTEndIndex() external {