diff --git a/Eigen/Core b/Eigen/Core
index 08044f2ec..3d325c716 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -13,7 +13,7 @@
 
 #ifndef EIGEN_DONT_VECTORIZE
   #define EIGEN_HAVE__SSE2__BUT_NOT_OLD_GCC ((defined __SSE2__) && ( (!defined __GNUC__) || EIGEN_GNUC_AT_LEAST(4,2) ))
-  #define EIGEN_HAVE_MSVC_SSE2 (defined(_M_IX86_FP) && (_M_IX86_FP >= 2))
+  #define EIGEN_HAVE_MSVC_SSE2 ((defined(_M_IX86_FP)) && (_M_IX86_FP >= 2)) // the parentheses around defined are really needed by MSVC
   #if (EIGEN_HAVE__SSE2__BUT_NOT_OLD_GCC || EIGEN_HAVE_MSVC_SSE2)
     #define EIGEN_VECTORIZE
     #define EIGEN_VECTORIZE_SSE