diff --git a/include/chibi/config.h b/include/chibi/config.h
index b070397b..a79c7b17 100644
--- a/include/chibi/config.h
+++ b/include/chibi/config.h
@@ -111,7 +111,7 @@
 #define strcasecmp cistrcmp
 #define strncasecmp cistrncmp
 #define round(x) floor((x)+0.5)
-#define trunc(x) ((x)-((x)-round(x)))
+#define trunc(x) floor((x)+0.5*(((x)<0)?1:0))
 
 #else