diff --git a/rts/ExternalAI/AICallback.cpp b/rts/ExternalAI/AICallback.cpp
index 527f3e958e7..57a1c907574 100644
--- a/rts/ExternalAI/AICallback.cpp
+++ b/rts/ExternalAI/AICallback.cpp
@@ -1822,12 +1822,12 @@ const float3* CAICallback::GetStartPos()
 
 
 
-#define AICALLBACK_CALL_LUA(HandleName)                                              \
-	const char* CAICallback::CallLua ## HandleName(const char* inData, int inSize) { \
-		if (lua ## HandleName == nullptr)                                            \
-			return nullptr;                                                             \
-                                                                                     \
-		return lua ## HandleName->RecvSkirmishAIMessage(team, inData, inSize);       \
+#define AICALLBACK_CALL_LUA(HandleName)                                                               \
+	const char* CAICallback::CallLua ## HandleName(const char* inData, int inSize, size_t* outSize) { \
+		if (lua ## HandleName == nullptr)                                                             \
+			return nullptr;                                                                           \
+                                                                                                      \
+		return lua ## HandleName->RecvSkirmishAIMessage(team, inData, inSize, outSize);               \
 	}
 
 AICALLBACK_CALL_LUA(Rules)
diff --git a/rts/ExternalAI/AICallback.h b/rts/ExternalAI/AICallback.h
index e0f7eaf7bf5..18aba228c39 100644
--- a/rts/ExternalAI/AICallback.h
+++ b/rts/ExternalAI/AICallback.h
@@ -255,8 +255,8 @@ class CAICallback
 	// 2. if inSize is less than 0, the data size is calculated using strlen()
 	// 3. the return data is subject to lua garbage collection,
 	//    copy it if you wish to continue using it
-	const char* CallLuaRules(const char* inData, int inSize = -1);
-	const char* CallLuaUI(const char* inData, int inSize = -1);
+	const char* CallLuaRules(const char* inData, int inSize, size_t* outSize);
+	const char* CallLuaUI(const char* inData, int inSize, size_t* outSize);
 
 	// never called, implemented in SSkirmishAICallbackImpl
 	std::map<std::string, std::string> GetMyInfo() { return std::map<std::string, std::string>(); }
diff --git a/rts/ExternalAI/SSkirmishAICallbackImpl.cpp b/rts/ExternalAI/SSkirmishAICallbackImpl.cpp
index 08157b5bc54..fd4cd70ce28 100644
--- a/rts/ExternalAI/SSkirmishAICallbackImpl.cpp
+++ b/rts/ExternalAI/SSkirmishAICallbackImpl.cpp
@@ -794,14 +794,15 @@ EXPORT(int) skirmishAiCallback_Engine_handleCommand(
 		#define SSAICALLBACK_CALL_LUA(HandleName, HANDLENAME)  \
 			case COMMAND_CALL_LUA_ ## HANDLENAME: {  \
 				SCallLua ## HandleName ## Command* cmd = static_cast<SCallLua ## HandleName ## Command*>(commandData);  \
-				const char* outData = clb->CallLua ## HandleName(cmd->inData, cmd->inSize);  \
-				if (cmd->ret_outData != NULL) {  \
-					if (outData != NULL) {  \
-						const int len = std::min(int(strlen(outData)), MAX_RESPONSE_SIZE - 1);  \
-						strncpy(cmd->ret_outData, outData, len);  \
-						cmd->ret_outData[len] = '\0';  \
-					} else {  \
-						cmd->ret_outData[0] = '\0';  \
+				size_t len = 0;                                                                                         \
+				const char* outData = clb->CallLua ## HandleName(cmd->inData, cmd->inSize, &len);                       \
+				if (cmd->ret_outData != NULL) {                                                                         \
+					if (outData != NULL) {                                                                              \
+						len = std::min(len, size_t(MAX_RESPONSE_SIZE - 1));                                             \
+						strncpy(cmd->ret_outData, outData, len);                                                        \
+						cmd->ret_outData[len] = '\0';                                                                   \
+					} else {                                                                                            \
+						cmd->ret_outData[0] = '\0';                                                                     \
 					}  \
 				}  \
 			} break;
diff --git a/rts/Lua/LuaHandle.cpp b/rts/Lua/LuaHandle.cpp
index e8de1ea7fe2..b05a07716f7 100644
--- a/rts/Lua/LuaHandle.cpp
+++ b/rts/Lua/LuaHandle.cpp
@@ -2360,7 +2360,7 @@ bool CLuaHandle::GameSetup(const string& state, bool& ready,
 
 
 
-const char* CLuaHandle::RecvSkirmishAIMessage(int aiTeam, const char* inData, int inSize)
+const char* CLuaHandle::RecvSkirmishAIMessage(int aiTeam, const char* inData, int inSize, size_t* outSize)
 {
 	LUA_CALL_IN_CHECK(L, nullptr);
 	luaL_checkstack(L, 4, __func__);
@@ -2389,7 +2389,7 @@ const char* CLuaHandle::RecvSkirmishAIMessage(int aiTeam, const char* inData, in
 		return nullptr;
 
 	if (lua_isstring(L, -1))
-		outData = lua_tolstring(L, -1, nullptr);
+		outData = lua_tolstring(L, -1, outSize);
 
 	lua_pop(L, 1);
 	return outData;
diff --git a/rts/Lua/LuaHandle.h b/rts/Lua/LuaHandle.h
index 07e5cb80d2c..2114791d87d 100644
--- a/rts/Lua/LuaHandle.h
+++ b/rts/Lua/LuaHandle.h
@@ -204,7 +204,7 @@ class CLuaHandle : public CEventClient
 		bool GameSetup(const std::string& state, bool& ready,
 		               const std::vector< std::pair<int, std::string> >& playerStates) override;
 
-		const char* RecvSkirmishAIMessage(int aiID, const char* data, int inSize);
+		const char* RecvSkirmishAIMessage(int aiID, const char* data, int inSize, size_t* outSize);
 
 		std::string WorldTooltip(const CUnit* unit,
 		                    const CFeature* feature,
diff --git a/rts/Lua/LuaRules.h b/rts/Lua/LuaRules.h
index 700443f6640..88f70cb00b1 100644
--- a/rts/Lua/LuaRules.h
+++ b/rts/Lua/LuaRules.h
@@ -37,8 +37,8 @@ class CLuaRules : public CSplitLuaHandle
 	void Cob2Lua(const LuaHashString& funcName, const CUnit* unit,
 	             int& argsCount, int args[MAX_LUA_COB_ARGS]);
 
-	const char* RecvSkirmishAIMessage(int aiID, const char* data, int inSize) {
-		return syncedLuaHandle.RecvSkirmishAIMessage(aiID, data, inSize);
+	const char* RecvSkirmishAIMessage(int aiID, const char* data, int inSize, size_t* outSize) {
+		return syncedLuaHandle.RecvSkirmishAIMessage(aiID, data, inSize, outSize);
 	}
 
 private: