diff --git a/pyadjoint/overloaded_type.py b/pyadjoint/overloaded_type.py
index 38ebeb74..75690781 100644
--- a/pyadjoint/overloaded_type.py
+++ b/pyadjoint/overloaded_type.py
@@ -1,3 +1,4 @@
+import weakref
 from .block_variable import BlockVariable
 from .tape import get_working_tape
 
@@ -64,6 +65,39 @@ def register_overloaded_type(overloaded_type, classes=None):
     return overloaded_type
 
 
+class Weakref:
+    """Weakref which is picklable if the referenced object is picklable or
+    None.
+
+    Args:
+        obj (:obj:`object`): The object to hold a weak reference to. None
+            indicates a reference to no object.
+    """
+
+    def __init__(self, obj=None):
+        self._init(obj)
+
+    def _init(self, obj):
+        if obj is None:
+            self._obj = lambda: None
+        else:
+            self._obj = weakref.ref(obj)
+
+    def __call__(self):
+        return self._obj()
+
+    def __getstate__(self):
+        state = self.__dict__.copy()
+        state["_obj"] = self()
+        return state
+
+    def __setstate__(self, state):
+        state = state.copy()
+        obj = state.pop("_obj")
+        self.__dict__.update(state)
+        self._init(obj)
+
+
 class OverloadedType(object):
     """Base class for OverloadedType types.
 
@@ -74,8 +108,7 @@ class OverloadedType(object):
     """
 
     def __init__(self, *args, **kwargs):
-        self.block_variable = None
-        self.create_block_variable()
+        self.clear_block_variable()
 
     @classmethod
     def _ad_init_object(cls, obj):
@@ -93,9 +126,21 @@ def _ad_init_object(cls, obj):
         """
         return cls(obj)
 
+    @property
+    def block_variable(self):
+        block_variable = self._block_variable()
+        return self.create_block_variable() if block_variable is None else block_variable
+
+    @block_variable.setter
+    def block_variable(self, value):
+        self._block_variable = Weakref(value)
+
+    def clear_block_variable(self):
+        self._block_variable = Weakref()
+
     def create_block_variable(self):
-        self.block_variable = BlockVariable(self)
-        return self.block_variable
+        self.block_variable = block_variable = BlockVariable(self)
+        return block_variable
 
     def _ad_convert_type(self, value, options={}):
         """This method must be overridden.
diff --git a/pyadjoint/tape.py b/pyadjoint/tape.py
index 64f3b2ba..9384d689 100644
--- a/pyadjoint/tape.py
+++ b/pyadjoint/tape.py
@@ -121,10 +121,10 @@ def __exit__(self, *args):
         _annotation_enabled = self._orig_annotation_enabled.pop()
         if self.modifies is not None:
             try:
-                self.modifies.create_block_variable()
+                self.modifies.clear_block_variable()
             except AttributeError:
                 for var in self.modifies:
-                    var.create_block_variable()
+                    var.clear_block_variable()
 
 
 no_annotations = stop_annotating()