Only allow serializing default units.

glue/core/state.py

@@ -65,6 +65,7 @@ def load(rec, context)
 import numpy as np
 from matplotlib.colors import Colormap
 from matplotlib import cm
+import astropy.units as u
 from astropy.units import UnitBase, Unit
 from astropy.wcs import WCS
 import shapely
@@ -624,7 +625,14 @@ def _load_slice(rec, context):
 
 @saver(UnitBase)
 def _save_unit_base(unit, context):
-    return dict(unit_base=unit.to_string())
+    unit_str = unit.to_string()
+    # Check that unit can be parsed back with default enabled systems
+    try:
+        with u.set_enabled_units([u.si, u.cgs, u.astrophys]):
+            _ = u.Unit(unit_str)
+    except ValueError:
+        raise GlueSerializeError(f"Serializing units of '{unit}' is not yet supported")
+    return dict(unit_base=unit_str)
 
 
 @loader(UnitBase)