diff --git a/src/codex32/generation.py b/src/codex32/generation.py
index 551ef61..db1843f 100644
--- a/src/codex32/generation.py
+++ b/src/codex32/generation.py
@@ -21,6 +21,7 @@
 )
 from codex32.errors import (
     CeremonyStateError,
+    CodexError,
     HeaderCollision,
     InvalidIdentifier,
     InvalidLength,
@@ -155,6 +156,8 @@ def generate_master_seed(
     """Generate or encode one unshared ``ms`` secret."""
     supplied, length = _seed_input(seed_bytes, byte_length)
     if supplied is not None:
+        if not _valid_root(supplied):
+            raise CodexError("master seed does not form a valid BIP32 root")
         identifier = _random_identifier() if identifier is None else _identifier(identifier)
         return MasterSeed.from_seed(supplied, identifier=identifier)
     if identifier is None and fingerprint is None:
diff --git a/tests/test_generation.py b/tests/test_generation.py
index 44cd688..289662b 100644
--- a/tests/test_generation.py
+++ b/tests/test_generation.py
@@ -25,6 +25,7 @@
 from codex32.bech32 import _u5_to_chars
 from codex32.errors import (
     CeremonyStateError,
+    CodexError,
     HeaderCollision,
     InvalidIdentifier,
     InvalidLength,
@@ -139,6 +140,12 @@ def test_raw_bytes_accept_random_or_explicit_identifiers() -> None:
     assert secret.header.identifier == "test"
 
 
+def test_supplied_seed_must_form_a_valid_bip32_root(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(generation_module, "_valid_root", lambda _seed: False)
+    with pytest.raises(CodexError, match="master seed does not form a valid BIP32 root"):
+        generate_master_seed(bytes(16), identifier="test")
+
+
 def test_explicit_and_random_output_order_contracts() -> None:
     source = generate_master_seed(bytes(range(16)), identifier="test")
     _secret, shares = _complete(