diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -48,6 +48,7 @@
         if not isinstance(d, dict):
             raise TypeError(
                 '%s.from_dict expects a dict, not %r' % (cls.__name__, d))
+        kwargs = {}
         for (name, attribute) in attr.fields_dict(cls).items():
             type_ = attribute.type
 
@@ -56,20 +57,19 @@
                 if name not in d:
                     continue
                 if d[name] is None:
-                    del d[name]
                     continue
                 else:
                     type_ = type_.__args__[0]
 
             # Construct an object of the expected type
             if issubclass(type_, BaseModel):
-                d[name] = type_.from_dict(d[name])
+                kwargs[name] = type_.from_dict(d[name])
             elif issubclass(type_, Enum):
-                d[name] = type_(d[name])
+                kwargs[name] = type_(d[name])
             else:
-                pass
+                kwargs[name] = d[name]
 
-        return cls(**d)
+        return cls(**kwargs)
 
 
 @attr.s
@@ -215,9 +215,12 @@
 
     @classmethod
     def from_dict(cls, d):
-        d['branches'] = {
-            name: SnapshotBranch.from_dict(branch)
-            for (name, branch) in d['branches'].items()
+        d = {
+            **d,
+            'branches': {
+                name: SnapshotBranch.from_dict(branch)
+                for (name, branch) in d['branches'].items()
+            }
         }
         return cls(**d)
 
@@ -306,7 +309,10 @@
 
     @classmethod
     def from_dict(cls, d):
-        d['entries'] = list(map(DirectoryEntry.from_dict, d['entries']))
+        d = {
+            **d,
+            'entries': list(map(DirectoryEntry.from_dict, d['entries']))
+        }
         return super().from_dict(d)
 
 
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -3,6 +3,8 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import copy
+
 from hypothesis import given
 
 from swh.model.hypothesis_strategies import objects
@@ -11,4 +13,14 @@
 @given(objects())
 def test_todict_inverse_fromdict(objtype_and_obj):
     (obj_type, obj) = objtype_and_obj
-    assert obj == type(obj).from_dict(obj.to_dict())
+    obj_as_dict = obj.to_dict()
+    obj_as_dict_copy = copy.deepcopy(obj_as_dict)
+
+    # Check the composition of to_dict and from_dict is the identity
+    assert obj == type(obj).from_dict(obj_as_dict)
+
+    # Check from_dict() does not change the input dict
+    assert obj_as_dict == obj_as_dict_copy
+
+    # Check the composition of from_dict and to_dict is the identity
+    assert obj_as_dict == type(obj).from_dict(obj_as_dict).to_dict()