diff --git a/src/string.cpp b/src/string.cpp
--- a/src/string.cpp
+++ b/src/string.cpp
@@ -26,7 +26,7 @@
 static int CDECL vseprintf(char *str, const char *last, const char *format, va_list ap)
 {
 	if (str >= last) return 0;
-	size_t size = last - str;
+	size_t size = last - str + 1;
 	return min((int)size, vsnprintf(str, size, format, ap));
 }