diff --git a/helper.pl b/helper.pl
index ffc592a7..8a430f73 100755
--- a/helper.pl
+++ b/helper.pl
@@ -249,7 +249,11 @@ sub make_sources_cmake {
   foreach my $hobj (sort @headers) {
     $output .= $hobj . "\n";
   }
-  $output .= ")\n";
+  $output .= ")
+
+if(MSVC AND BUILD_SHARED_LIBS)
+  list(APPEND SOURCES tommath.def)
+endif()\n";
   return $output;
 }
 
diff --git a/sources.cmake b/sources.cmake
index e3c12433..ba3012ef 100644
--- a/sources.cmake
+++ b/sources.cmake
@@ -173,3 +173,7 @@ tommath_cutoffs.h
 tommath_private.h
 tommath_superclass.h
 )
+
+if(MSVC AND BUILD_SHARED_LIBS)
+  list(APPEND SOURCES tommath.def)
+endif()