diff --git a/examples/ClientTests/src/NestedBoxReflection_geo.cpp b/examples/ClientTests/src/NestedBoxReflection_geo.cpp
index e38b6b7559fa2ab4630e85cbf67a589e15727fac..9c4e933f011ce7e2b0761917012584ef428d5309 100644
--- a/examples/ClientTests/src/NestedBoxReflection_geo.cpp
+++ b/examples/ClientTests/src/NestedBoxReflection_geo.cpp
@@ -36,7 +36,7 @@ namespace   {
 
     constexpr double tol = 1.0e-3;       // Geant4 compatible
     double check = (x.Cross(y)).Dot(z);  // in case of a LEFT-handed orthogonal system this must be -1
-    if (abs(1. + check) > tol) {
+    if (std::abs(1. + check) > tol) {
       except("NestedBoxReflection", "+++ FAILED to construct Rotation is not LEFT-handed!");
     }
     printout(INFO, "NestedBoxReflection", "+++ Constructed LEFT-handed reflection rotation.");