diff --git a/src/value.c b/src/value.c
index 438a4de..333d0af 100644
--- a/src/value.c
+++ b/src/value.c
@@ -29,7 +29,7 @@
 
 /* Work around nonstandard isnan() and isinf() implementations */
 #ifndef isnan
-#ifndef sun
+#ifndef __sun
 static JSON_INLINE int isnan(double x) { return x != x; }
 #endif
 #endif