diff --git a/contrib/Netgen/libsrc/gprim/transform3d.cpp b/contrib/Netgen/libsrc/gprim/transform3d.cpp
index ea62fffe5a0e9563644b0428f822ca1bbf2cac77..37c17b220083f6142bf13da2fc26a8399b3f1173 100644
--- a/contrib/Netgen/libsrc/gprim/transform3d.cpp
+++ b/contrib/Netgen/libsrc/gprim/transform3d.cpp
@@ -31,6 +31,7 @@ Transformation3d :: Transformation3d (const Vec3d & translate)
     }
 }
 
+void CalcInverse (const DenseMatrix & m1, DenseMatrix & m2);
 
 Transformation3d :: 
 Transformation3d (const Point3d & c, double alpha,