diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 797428a..bed7229 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -489,7 +489,7 @@
 		if (str < end)
 			*str = '\0';
 		else
-			*end = '\0';
+			end[-1] = '\0';
 	}
 	/* the trailing null byte doesn't count towards the total */
 	return str-buf;