diff --git a/lib/crypto/siphash.js b/lib/crypto/siphash.js
index 66160234..433de697 100644
--- a/lib/crypto/siphash.js
+++ b/lib/crypto/siphash.js
@@ -153,14 +153,21 @@ function U64(hi, lo) {
 
 U64.prototype.iadd = function iadd(b) {
   var a = this;
-  var sum, c;
+  var hi, lo, as, bs, s, c;
 
   // Credit to @indutny for this method.
-  sum = (a.lo >>> 0) + (b.lo >>> 0);
-  c = (sum >= 0x100000000) | 0;
+  lo = (a.lo + b.lo) | 0;
 
-  a.hi = (((a.hi + b.hi) | 0) + c) | 0;
-  a.lo = sum | 0;
+  s = lo >> 31;
+  as = a.lo >> 31;
+  bs = b.lo >> 31;
+
+  c = ((as & bs) | (~s & (as ^ bs))) & 1;
+
+  hi = ((a.hi + b.hi) | 0) + c;
+
+  a.hi = hi | 0;
+  a.lo = lo;
 
   return a;
 };