diff --git a/src/Mod/PartDesign/App/DatumCS.cpp b/src/Mod/PartDesign/App/DatumCS.cpp
index 05c7b4e17..591e32fc6 100644
--- a/src/Mod/PartDesign/App/DatumCS.cpp
+++ b/src/Mod/PartDesign/App/DatumCS.cpp
@@ -542,7 +542,7 @@ void CoordinateSystem::onChanged(const App::Property *prop)
         else if(line1) {
             
             if(line2) {
-                if(! ((lin1.Angle(lin2) - M_PI<2) < Precision::Angular()) )
+                if(! (fabs(lin1.Angle(lin2) - M_PI/2.0) < Precision::Angular()) )
                         throw Base::Exception("Lines must be perpendicular");
                 ax = gp_Ax3(lin1.Location(), lin1.Direction(), lin2.Direction());
             }
@@ -608,4 +608,4 @@ Base::Vector3d CoordinateSystem::getZAxis()
     Base::Vector3d normal;
     rot.multVec(Base::Vector3d(0,0,1), normal);
     return normal;
-}
\ No newline at end of file
+}