diff --git a/auditlog/diff.py b/auditlog/diff.py
index 51cf5c3..bd5fe59 100644
--- a/auditlog/diff.py
+++ b/auditlog/diff.py
@@ -206,10 +206,10 @@ def model_instance_diff(
 
         if old_value != new_value:
             if field.name in custom_fields_callbacks:
-                old_value = " FIELD TOO LARGE "
                 new_value = custom_fields_callbacks[field.name](
-                    old_value, new_value
+                    json.dumps(old_value), json.dumps(new_value)
                 )
+                old_value = " FIELD TOO LARGE "
                 diff[field.name] = (smart_str(old_value), smart_str(new_value))
             elif model_fields and field.name in model_fields["mask_fields"]:
                 diff[field.name] = (
diff --git a/setup.py b/setup.py
index 901ab66..5927137 100644
--- a/setup.py
+++ b/setup.py
@@ -28,7 +28,11 @@ setup(
     long_description=long_description,
     long_description_content_type="text/markdown",
     python_requires=">=3.8",
-    install_requires=["Django>=3.2", "python-dateutil>=2.7.0"],
+    install_requires=[
+        "Django>=3.2",
+        "python-dateutil>=2.7.0",
+        "deepdiff==7.0.*",
+    ],
     zip_safe=False,
     classifiers=[
         "Programming Language :: Python :: 3",