--- a/src/rrd_format.h
+++ b/src/rrd_format.h
@@ -20,7 +20,7 @@
 
 #define RRD_COOKIE    "RRD"
 #define RRD_VERSION   "0001"
-#define FLOAT_COOKIE  8.642135E130
+#define FLOAT_COOKIE  ((double)8.642135E130)
 
 #if defined(WIN32)
 #define DNAN          ((double)fmod(0.0,0.0))