diff --git a/glue_jupyter/common/tests/__init__.py b/glue_jupyter/common/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/glue_jupyter/common/tests/test_state3d.py b/glue_jupyter/common/tests/test_state3d.py
new file mode 100644
index 00000000..2c32f660
--- /dev/null
+++ b/glue_jupyter/common/tests/test_state3d.py
@@ -0,0 +1,35 @@
+import traitlets
+
+from glue_jupyter.state_traitlets_helpers import GlueState
+from glue_jupyter.common.state3d import ViewerState3D
+
+
+class Widget(traitlets.HasTraits):
+
+    state = GlueState()
+
+    latest_json = None
+
+    # The following two methods mimic the behavior of ipywidgets
+
+    @traitlets.observe('state')
+    def on_state_change(self, change):
+        to_json = self.trait_metadata('state', 'to_json')
+        self.latest_json = to_json(self.state, self)
+
+    def set_state_from_json(self, json):
+        from_json = self.trait_metadata('state', 'from_json')
+        from_json(json, self)
+
+
+def test_json_serializable():
+    widget = Widget()
+    assert widget.latest_json is None
+    widget.state = ViewerState3D()
+    assert widget.latest_json == {
+        "x_att": None, "x_min": 0, "x_max": 1,
+        "y_att": None, "y_min": 0, "y_max": 1,
+        "z_att": None, "z_min": 0, "z_max": 1,
+        "visible_axes": True, "native_aspect": False,
+        "layers": [], "title": None,
+    }