diff --git a/circuits/merkleTree.circom b/circuits/merkleTree.circom
index 1764f14..ce8862d 100644
--- a/circuits/merkleTree.circom
+++ b/circuits/merkleTree.circom
@@ -20,6 +20,7 @@ template DualMux() {
     signal input s;
     signal output out[2];
 
+    s * (1 - s) === 0
     out[0] <== (in[1] - in[0])*s + in[0];
     out[1] <== (in[0] - in[1])*s + in[1];
 }