--- a/main/utils.c
+++ b/main/utils.c
@@ -1756,7 +1756,7 @@ void __ast_string_field_ptr_build_va(str
 		available = space - sizeof(ast_string_field_allocation);
 	}
 
-	needed = vsnprintf(target, available, format, ap1) + 1;
+	needed = vsnprintf(target, available, format, ap1) + 2;
 
 	va_end(ap1);