diff --git a/odoo/fields.py b/odoo/fields.py
index e6a6d1df1ee..7b3cf6e3a41 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -638,9 +638,8 @@ class Field(MetaField('DummyField', (object,), {})):
 
     def cache_key(self, env):
         """ Return the cache key corresponding to ``self.depends_context``. """
-        get_context = env.context.get
 
-        def get(key):
+        def get(key, get_context=env.context.get):
             if key == 'company':
                 return env.company.id
             elif key == 'uid':
@@ -648,7 +647,16 @@ class Field(MetaField('DummyField', (object,), {})):
             elif key == 'active_test':
                 return get_context('active_test', self.context.get('active_test', True))
             else:
-                return get_context(key)
+                v = get_context(key)
+                try: hash(v)
+                except TypeError:
+                    raise TypeError(
+                        "Can only create cache keys from hashable values, "
+                        "got non-hashable value {!r} at context key {!r} "
+                        "(dependency of field {})".format(v, key, self)
+                    ) from None # we don't need to chain the exception created 2 lines above
+                else:
+                    return v
 
         return tuple(get(key) for key in self.depends_context)