diff --git a/Lib/email/quoprimime.py b/Lib/email/quoprimime.py
index 78638d5904c..bc02281b1d7 100644
--- a/Lib/email/quoprimime.py
+++ b/Lib/email/quoprimime.py
@@ -319,4 +319,4 @@ def header_decode(s):
     the high level email.header class for that functionality.
     """
     s = s.replace('_', ' ')
-    return re.sub(r'=[a-fA-F0-9]{2}', _unquote_match, s, re.ASCII)
+    return re.sub(r'=[a-fA-F0-9]{2}', _unquote_match, s, flags=re.ASCII)
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 78b86b8630d..87579245227 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -4048,6 +4048,10 @@ class TestQuopri(unittest.TestCase):
     def test_header_decode_non_ascii(self):
         self._test_header_decode('hello=C7there', 'hello\xc7there')
 
+    def test_header_decode_re_bug_18380(self):
+        # Issue 18380: Call re.sub with a positional argument for flags in the wrong position
+        self.assertEqual(quoprimime.header_decode('=30' * 257), '0' * 257)
+
     def _test_decode(self, encoded, expected_decoded, eol=None):
         if eol is None:
             decoded = quoprimime.decode(encoded)
diff --git a/Misc/ACKS b/Misc/ACKS
index 71402b71ac1..cf64ed2ea35 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -849,6 +849,7 @@ Alessandro Moura
 Pablo Mouzo
 Mher Movsisyan
 Ruslan Mstoi
+Valentina Mukhamedzhanova
 Michael Mulich
 Sape Mullender
 Sjoerd Mullender