diff --git a/src/main.cpp b/src/main.cpp
index 2fb4603..9b9ef95 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -74,7 +74,7 @@ std::string argsToShort(std::vector<SRequestArgument>& args, const std::string&
         else if (a.wlType == "fixed")
             shortt += "f";
         else if (a.wlType == "string")
-            shortt += "s";
+            shortt += std::string(a.allowNull ? "?s" : "s");
         else if (a.wlType == "object")
             shortt += std::string(a.allowNull ? "?" : "") + "o";
         else if (a.wlType == "array")