diff --git a/a8/vec2.cc b/a8/vec2.cc
index 6e76997..9c76cc0 100644
--- a/a8/vec2.cc
+++ b/a8/vec2.cc
@@ -92,7 +92,7 @@ namespace a8
         return vec2;
     }
 
-    float Vec2::Distance(const Vec2& b)
+    float Vec2::Distance(const Vec2& b) const
     {
         Vec2 v = b - *this;
         return v.Norm();
diff --git a/a8/vec2.h b/a8/vec2.h
index b9370be..8f93654 100644
--- a/a8/vec2.h
+++ b/a8/vec2.h
@@ -15,7 +15,7 @@ namespace a8
         float CalcAngle(const Vec2& b) const;
         float CalcAngleEx(const Vec2& b) const;
         static Vec2 FromAngle(float angle);
-        float Distance(const Vec2& b);
+        float Distance(const Vec2& b) const;
         float ManhattanDistance(const Vec2& b);
 
         bool operator == (const Vec2& b) const;