diff --git a/test/boostmultiprec.cpp b/test/boostmultiprec.cpp
index 3e16aeabd..b4d0decff 100644
--- a/test/boostmultiprec.cpp
+++ b/test/boostmultiprec.cpp
@@ -72,6 +72,7 @@ namespace Eigen {
   template<>
   Real test_precision<Real>() { return 1e-50; }
 
+  // needed in C++93 mode where number does not support explicit cast.
   namespace internal {
     template<typename NewType>
     struct cast_impl<Real,NewType>