diff --git a/tuf/api/dsse.py b/tuf/api/dsse.py
index d027d14013..493fefd1d0 100644
--- a/tuf/api/dsse.py
+++ b/tuf/api/dsse.py
@@ -81,7 +81,7 @@ def from_bytes(cls, data: bytes) -> SimpleEnvelope[T]:
         except Exception as e:
             raise DeserializationError from e
 
-        return envelope
+        return cast(SimpleEnvelope[T], envelope)
 
     def to_bytes(self) -> bytes:
         """Return envelope as JSON bytes.
diff --git a/tuf/api/serialization/json.py b/tuf/api/serialization/json.py
index a031ef8255..dcff79e029 100644
--- a/tuf/api/serialization/json.py
+++ b/tuf/api/serialization/json.py
@@ -98,7 +98,10 @@ def serialize(self, signed_obj: Signed) -> bytes:
         """
         try:
             signed_dict = signed_obj.to_dict()
-            canonical_bytes = encode_canonical(signed_dict).encode("utf-8")
+            canon_str = encode_canonical(signed_dict)
+            # encode_canonical cannot return None if output_function is not set
+            assert canon_str is not None  # noqa: S101
+            canonical_bytes = canon_str.encode("utf-8")
 
         except Exception as e:
             raise SerializationError from e