diff --git a/src/eval.c b/src/eval.c
index cf908d1..b04b6c2 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -27,8 +27,14 @@
 # include <time.h>	/* for time_t */
 #endif
 
-#if defined(FEAT_FLOAT) && defined(HAVE_MATH_H)
-# include <math.h>
+#if defined(FEAT_FLOAT)
+# include <float.h>
+# if defined(HAVE_MATH_H)
+#  include <math.h>
+# endif
+# if defined(WIN32) && !defined(isnan)
+#  define isnan(x) _isnan(x)
+# endif
 #endif
 
 #define DICT_MAXNEST 100	/* maximum nesting of lists and dicts */