diff --git a/calculator-backends.js b/calculator-backends.js
index 86395f4..d0b10f6 100644
--- a/calculator-backends.js
+++ b/calculator-backends.js
@@ -57,7 +57,7 @@ function gcd(a, b) {
 function Fraction(n, d) {
     if (d === undefined)
         d = new BigInteger(1);
-    var x = gcd(n, d);
+    var x = gcd(n.abs(), d);
     this.n = n.divide(x);
     this.d = d.divide(x);
 }