diff --git a/CHANGES.rst b/CHANGES.rst
index be6a7341..9545eb06 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -8,7 +8,9 @@
 Bug fixes
 ---------
 
-- Ensure headers words are never turned into signed 32-bit numpy integers.
+- Ensure headers words are never turned into signed 32- or 64-bit numpy
+  integers. [#538, #540]
+
 
 4.3 (2025-03-10)
 ================
diff --git a/baseband/base/header.py b/baseband/base/header.py
index 84a5711b..6204df9a 100644
--- a/baseband/base/header.py
+++ b/baseband/base/header.py
@@ -118,12 +118,18 @@ def setter(words, value):
                 raise ValueError("no default value so cannot set to 'None'.")
             value = default
         elif value is True:
+            # Special case: may set multiple bits in invariant masks.
             value = bit_mask
-        elif np.any(value & bit_mask != value):
-            raise ValueError("{0} cannot be represented with {1} bits"
-                             .format(value, bit_length))
+        else:
+            if isinstance(value, (np.integer, np.bool_)):
+                # Turn into integer to avoid influencing promotion.
+                value = int(value)
+            if np.any(value & bit_mask != value):
+                raise ValueError("{0} cannot be represented with {1} bits"
+                                 .format(value, bit_length))
+
         if bit_length == 64:
-            word1 = value & (1 << 32) - 1
+            word1 = value & 0xffffffff
             word2 = value >> 32
             words[word_index] = word1
             words[word_index + 1] = word2
diff --git a/baseband/mark4/header.py b/baseband/mark4/header.py
index 7ef2afda..5bd5193a 100644
--- a/baseband/mark4/header.py
+++ b/baseband/mark4/header.py
@@ -218,7 +218,7 @@ def fraction(self, fraction):
             raise ValueError("{0} ms is not a multiple of 1.25 ms"
                              .format(ms))
         self['bcd_fraction'] = bcd_encode(np.floor(ms + 1e-6)
-                                          .astype(np.int32))
+                                          .astype(np.uint32))
 
     def get_time(self):
         """Convert BCD time code to Time object.
diff --git a/baseband/mark4/tests/test_mark4.py b/baseband/mark4/tests/test_mark4.py
index af46381c..f0437b7f 100644
--- a/baseband/mark4/tests/test_mark4.py
+++ b/baseband/mark4/tests/test_mark4.py
@@ -224,6 +224,14 @@ def test_header(self, tmpdir):
         assert header13.ntrack == 53
         assert abs(header13.time - header.time) < 1. * u.ns
 
+    @pytest.mark.parametrize("value", [1, np.int32(1), np.int64(1)])
+    def test_header_word_type(self, value):
+        # Regression test for numpy scalars influencing the type of the words.
+        header = Mark4TrackHeader(words=None)
+        header["converter_id"] = value
+        assert isinstance(header.words, list)
+        assert all(type(w) is int for w in header.words)
+
     def test_invariant_pattern(self):
         with open(SAMPLE_FILE, 'rb') as fh:
             fh.seek(0xa88)
diff --git a/docs/nitpick-exceptions b/docs/nitpick-exceptions
index 9c4ba40c..966881ef 100644
--- a/docs/nitpick-exceptions
+++ b/docs/nitpick-exceptions
@@ -23,6 +23,7 @@ py:class a set-like object providing a view on D's items
 py:class a set-like object providing a view on D's keys
 py:class v, remove specified key and return the corresponding value.
 py:class None.  Update D from dict/iterable E and F.
+py:class None.  Update D from mapping/iterable E and F.
 py:class an object providing a view on D's values
 py:class a shallow copy of D
 # Same but for OrderedDict (used in DADAHeader).