diff --git a/src/codex32/bip93.py b/src/codex32/bip93.py
index b73a3ff..9e3757c 100644
--- a/src/codex32/bip93.py
+++ b/src/codex32/bip93.py
@@ -47,7 +47,11 @@ class Header:
     index: str
 
     def __post_init__(self) -> None:
-        if isinstance(self.threshold, bool) or self.threshold not in (0, *range(2, 10)):
+        if (
+            isinstance(self.threshold, bool)
+            or not isinstance(self.threshold, int)
+            or self.threshold not in (0, *range(2, 10))
+        ):
             raise InvalidThreshold("threshold must be 0 or an integer from 2 through 9")
         if not isinstance(self.identifier, str):
             raise InvalidIdentifier("identifier must be str")
diff --git a/tests/test_public_api.py b/tests/test_public_api.py
index 12f906a..09d66d4 100644
--- a/tests/test_public_api.py
+++ b/tests/test_public_api.py
@@ -115,6 +115,7 @@ def test_master_seed_factory_can_only_construct_index_s() -> None:
     ("arguments", "error"),
     (
         ((1, "test", "s"), InvalidThreshold),
+        ((2.0, "test", "a"), InvalidThreshold),
         ((2, "bad", "a"), InvalidIdentifier),
         ((0, "test", "a"), InvalidShareIndex),
     ),