diff --git a/src/emerge.cpp b/src/emerge.cpp
index bb2590641..92edd2007 100644
--- a/src/emerge.cpp
+++ b/src/emerge.cpp
@@ -256,6 +256,12 @@ void EmergeManager::stopThreads()
 }
 
 
+bool EmergeManager::isRunning()
+{
+	return m_threads_active;
+}
+
+
 bool EmergeManager::enqueueBlockEmerge(
 	u16 peer_id,
 	v3s16 blockpos,
diff --git a/src/emerge.h b/src/emerge.h
index b72b2a2c3..47ff218b8 100644
--- a/src/emerge.h
+++ b/src/emerge.h
@@ -115,6 +115,7 @@ public:
 
 	void startThreads();
 	void stopThreads();
+	bool isRunning();
 
 	bool enqueueBlockEmerge(
 		u16 peer_id,
diff --git a/src/script/lua_api/l_mapgen.cpp b/src/script/lua_api/l_mapgen.cpp
index 44af7892f..cfa216a3d 100644
--- a/src/script/lua_api/l_mapgen.cpp
+++ b/src/script/lua_api/l_mapgen.cpp
@@ -619,7 +619,11 @@ int ModApiMapgen::l_set_mapgen_params(lua_State *L)
 	if (!lua_istable(L, 1))
 		return 0;
 
-	MapgenParams *params = &getServer(L)->getEmergeManager()->params;
+	EmergeManager *emerge = getServer(L)->getEmergeManager();
+	if (emerge->isRunning())
+		throw LuaError("Cannot set parameters while mapgen is running");
+
+	MapgenParams *params = &emerge->params;
 	u32 flags = 0, flagmask = 0;
 
 	lua_getfield(L, 1, "mgname");
@@ -637,6 +641,10 @@ int ModApiMapgen::l_set_mapgen_params(lua_State *L)
 	if (lua_isnumber(L, -1))
 		params->water_level = lua_tointeger(L, -1);
 
+	lua_getfield(L, 1, "chunksize");
+	if (lua_isnumber(L, -1))
+		params->chunksize = lua_tointeger(L, -1);
+
 	warn_if_field_exists(L, 1, "flagmask",
 		"Deprecated: flags field now includes unset flags.");
 	lua_getfield(L, 1, "flagmask");