diff --git a/msvc/ecl/config.h.msvc6 b/msvc/ecl/config.h.msvc6
index d25483d24..cca19c96b 100644
--- a/msvc/ecl/config.h.msvc6
+++ b/msvc/ecl/config.h.msvc6
@@ -319,3 +319,8 @@ typedef unsigned int uint32_t;
 #define sleep _sleep
 
 #include "@ECL_FPE_CODE@"
+
+#include <math.h>
+#ifndef isfinite
+#define isfinite(x) (finite(x))
+#endif