diff --git a/Geo/MPrism.h b/Geo/MPrism.h
index 3ff52cb36fad87ab433e3be1aea1f5eb6de612bf..4b3713ee09bf0b554b4f263e3e6d8c19007015ab 100644
--- a/Geo/MPrism.h
+++ b/Geo/MPrism.h
@@ -168,8 +168,8 @@ class MPrism : public MElement {
   virtual bool isInside(double u, double v, double w)
   {
     double tol = _isInsideTolerance;
-    if(w > (1. + tol) || w < -(1. + tol) || u < (1. + tol)
-       || v < (1. + tol) || u > ((1. + tol) - v))
+    if(w > (1. + tol) || w < -(1. + tol) || u < (-tol)
+       || v < (-tol) || u > ((1. + tol) - v))
       return false;
     return true;
   }