diff --git a/plugins/3MFWriter/ThreeMFWorkspaceWriter.py b/plugins/3MFWriter/ThreeMFWorkspaceWriter.py
index 08918e1c12..10a21f445b 100644
--- a/plugins/3MFWriter/ThreeMFWorkspaceWriter.py
+++ b/plugins/3MFWriter/ThreeMFWorkspaceWriter.py
@@ -80,7 +80,8 @@ class ThreeMFWorkspaceWriter(WorkspaceWriter):
         mesh_writer.setStoreArchive(False)
         return True
 
-    def _writePluginMetadataToArchive(self, archive):
+    @staticmethod
+    def _writePluginMetadataToArchive(archive: zipfile.ZipFile) -> None:
         file_name_template = "%s/plugin_metadata.json"
 
         for plugin_id, metadata in Application.getInstance().getWorkspaceMetadataStorage().getAllData().items():
diff --git a/plugins/3MFWriter/ThreeMFWriter.py b/plugins/3MFWriter/ThreeMFWriter.py
index 640aabd30c..9860804542 100644
--- a/plugins/3MFWriter/ThreeMFWriter.py
+++ b/plugins/3MFWriter/ThreeMFWriter.py
@@ -1,5 +1,6 @@
 # Copyright (c) 2015 Ultimaker B.V.
 # Uranium is released under the terms of the LGPLv3 or higher.
+from typing import Optional
 
 from UM.Mesh.MeshWriter import MeshWriter
 from UM.Math.Vector import Vector
@@ -40,7 +41,7 @@ class ThreeMFWriter(MeshWriter):
         }
 
         self._unit_matrix_string = self._convertMatrixToString(Matrix())
-        self._archive = None
+        self._archive = None  # type: Optional[zipfile.ZipFile]
         self._store_archive = False
 
     def _convertMatrixToString(self, matrix):