diff --git a/data/multiplayer/scenarios/Random_Scenario_Cave.cfg b/data/multiplayer/scenarios/Random_Scenario_Cave.cfg
index e6c3b71cb3f2..b36130502ec7 100644
--- a/data/multiplayer/scenarios/Random_Scenario_Cave.cfg
+++ b/data/multiplayer/scenarios/Random_Scenario_Cave.cfg
@@ -177,5 +177,6 @@ Rb,Rb,Rb,Rb,Rb,Rb,Rb,Rb^Uf,Rb^Ii,Sm,Sm,Uue,Rb^Fetd,Rb^Fdw#enddef
     [/generator]
 [/multiplayer]
 
+#undef CLEAR_TERRAINS
 #undef ROAD_COSTS
 #undef PLAYER_CHAMBER
diff --git a/src/scripting/mapgen_lua_kernel.cpp b/src/scripting/mapgen_lua_kernel.cpp
index 732106f6e180..e531edc5c8e4 100644
--- a/src/scripting/mapgen_lua_kernel.cpp
+++ b/src/scripting/mapgen_lua_kernel.cpp
@@ -152,9 +152,15 @@ void mapgen_lua_kernel::run_generator(const char * prog, const config & generato
 	protected_call(1, 1, std::bind(&lua_kernel_base::throw_exception, this, _1, _2));
 }
 
-void mapgen_lua_kernel::user_config(const char * prog, const config & generator)
+void mapgen_lua_kernel::user_config(const char * prog, config & generator)
 {
 	run_generator(prog, generator);
+	if(!luaW_toconfig(mState, -1, generator)) {
+		std::string msg = "expected a string, found a ";
+		msg += lua_typename(mState, lua_type(mState, -1));
+		lua_pop(mState, 1);
+		throw game::lua_error(msg.c_str(),"bad return value");
+	}
 }
 
 std::string mapgen_lua_kernel::create_map(const char * prog, const config & generator, boost::optional<uint32_t> seed) // throws game::lua_error
diff --git a/src/scripting/mapgen_lua_kernel.hpp b/src/scripting/mapgen_lua_kernel.hpp
index 038f72e91d98..a995dcfb29aa 100644
--- a/src/scripting/mapgen_lua_kernel.hpp
+++ b/src/scripting/mapgen_lua_kernel.hpp
@@ -29,7 +29,7 @@ class mapgen_lua_kernel : public lua_kernel_base {
 
 	virtual std::string my_name() { return "Mapgen Lua Kernel"; }
 
-	void user_config(const char * prog, const config & generator); // throws game::lua_error
+	void user_config(const char * prog, config & generator); // throws game::lua_error
 	std::string create_map(const char * prog, const config & generator, boost::optional<uint32_t> seed); // throws game::lua_error
 	config create_scenario(const char * prog, const config & generator, boost::optional<uint32_t> seed); // throws game::lua_error