diff --git a/lib/BasicTypes.h b/lib/BasicTypes.h
index 18097d20c..8907ec45d 100644
--- a/lib/BasicTypes.h
+++ b/lib/BasicTypes.h
@@ -177,7 +177,7 @@ class XYZ {
   }
 
   double LengthSq() const { return x * x + y * y + z * z; }
-  double Length() const { return sqrt(LengthSq()); }
+  double Length() const { return std::sqrt(LengthSq()); }
 
   XYZ &Normalize() {
     *this *= (1.0 / Length());