diff --git a/src/objectBox.cc b/src/objectBox.cc
index 13cc574..70e880b 100644
--- a/src/objectBox.cc
+++ b/src/objectBox.cc
@@ -229,7 +229,7 @@ Box::compute_normal(const Vector3& p)
         return Vector3(0, 1, 0);
     } else if (NearlyEqual(p.z, mNear.z)) {
         return Vector3(0, 0, -1);
-    } else if (NearlyEqual(p.z, mFar.y)) {
+    } else if (NearlyEqual(p.z, mFar.z)) {
         return Vector3(0, 0, 1);
     }