diff --git a/math/sqrt_double.cpp b/math/sqrt_double.cpp
index 6d7641dcb..e3418bfc3 100644
--- a/math/sqrt_double.cpp
+++ b/math/sqrt_double.cpp
@@ -12,6 +12,7 @@ double Sqrt(double x) {
     between 1e-7 and 1e-12.
     double epsilon = 1e-12;
     */
+    double epsilon = 1e-12;
     while ( l <= r ) {
         double mid = (l + r) / 2;
         if ( mid * mid > x ) {