diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -18,6 +18,7 @@
 from .collections import ImmutableDict
 from .hashutil import DEFAULT_ALGORITHMS, MultiHash, hash_to_bytes
 from .identifiers import (
+    _BaseSWHID,
     directory_identifier,
     extid_identifier,
     normalize_timestamp,
@@ -66,16 +67,19 @@
 
 def dictify(value):
     "Helper function used by BaseModel.to_dict()"
-    if isinstance(value, BaseModel):
+    if isinstance(value, (int, float, str, bytes)):
+        # short-path for common objects
+        return value
+    elif hasattr(value, "to_dict"):  # model object
         return value.to_dict()
-    elif isinstance(value, (CoreSWHID, ExtendedSWHID)):
-        return str(value)
+    elif hasattr(value, "items"):  # dict
+        return {k: dictify(v) for k, v in value.items()}
+    elif hasattr(value, "__iter__"):  # tuple
+        return tuple(map(dictify, value))
     elif isinstance(value, Enum):
         return value.value
-    elif isinstance(value, (dict, ImmutableDict)):
-        return {k: dictify(v) for k, v in value.items()}
-    elif isinstance(value, tuple):
-        return tuple(dictify(v) for v in value)
+    elif isinstance(value, _BaseSWHID):
+        return str(value)
     else:
         return value