diff --git a/Lib/encodings/base64_codec.py b/Lib/encodings/base64_codec.py
index 8e7703b3b6072d..e740d2bd1c8a1c 100644
--- a/Lib/encodings/base64_codec.py
+++ b/Lib/encodings/base64_codec.py
@@ -11,11 +11,13 @@
 ### Codec APIs
 
 def base64_encode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     return (base64.encodebytes(input), len(input))
 
 def base64_decode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     return (base64.decodebytes(input), len(input))
 
 class Codec(codecs.Codec):
diff --git a/Lib/encodings/bz2_codec.py b/Lib/encodings/bz2_codec.py
index fd9495e341baee..2f732d9344ac75 100644
--- a/Lib/encodings/bz2_codec.py
+++ b/Lib/encodings/bz2_codec.py
@@ -10,14 +10,20 @@
 import codecs
 import bz2 # this codec needs the optional bz2 module !
 
+### Codec Helpers
+
+def _assert_strict(errors):
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
+
 ### Codec APIs
 
 def bz2_encode(input, errors='strict'):
-    assert errors == 'strict'
+    _assert_strict(errors)
     return (bz2.compress(input), len(input))
 
 def bz2_decode(input, errors='strict'):
-    assert errors == 'strict'
+    _assert_strict(errors)
     return (bz2.decompress(input), len(input))
 
 class Codec(codecs.Codec):
@@ -28,7 +34,7 @@ def decode(self, input, errors='strict'):
 
 class IncrementalEncoder(codecs.IncrementalEncoder):
     def __init__(self, errors='strict'):
-        assert errors == 'strict'
+        _assert_strict(errors)
         self.errors = errors
         self.compressobj = bz2.BZ2Compressor()
 
@@ -44,7 +50,7 @@ def reset(self):
 
 class IncrementalDecoder(codecs.IncrementalDecoder):
     def __init__(self, errors='strict'):
-        assert errors == 'strict'
+        _assert_strict(errors)
         self.errors = errors
         self.decompressobj = bz2.BZ2Decompressor()
 
diff --git a/Lib/encodings/hex_codec.py b/Lib/encodings/hex_codec.py
index 9fb1072804456a..ae227833a80083 100644
--- a/Lib/encodings/hex_codec.py
+++ b/Lib/encodings/hex_codec.py
@@ -11,11 +11,13 @@
 ### Codec APIs
 
 def hex_encode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     return (binascii.b2a_hex(input), len(input))
 
 def hex_decode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     return (binascii.a2b_hex(input), len(input))
 
 class Codec(codecs.Codec):
diff --git a/Lib/encodings/quopri_codec.py b/Lib/encodings/quopri_codec.py
index 496cb7655d032d..9af22662881f83 100644
--- a/Lib/encodings/quopri_codec.py
+++ b/Lib/encodings/quopri_codec.py
@@ -8,14 +8,16 @@
 from io import BytesIO
 
 def quopri_encode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     f = BytesIO(input)
     g = BytesIO()
     quopri.encode(f, g, quotetabs=True)
     return (g.getvalue(), len(input))
 
 def quopri_decode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     f = BytesIO(input)
     g = BytesIO()
     quopri.decode(f, g)
diff --git a/Lib/encodings/uu_codec.py b/Lib/encodings/uu_codec.py
index 4e58c62fe9ef0f..4e9b57c8de9c84 100644
--- a/Lib/encodings/uu_codec.py
+++ b/Lib/encodings/uu_codec.py
@@ -14,7 +14,8 @@
 ### Codec APIs
 
 def uu_encode(input, errors='strict', filename='<data>', mode=0o666):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     infile = BytesIO(input)
     outfile = BytesIO()
     read = infile.read
@@ -35,7 +36,8 @@ def uu_encode(input, errors='strict', filename='<data>', mode=0o666):
     return (outfile.getvalue(), len(input))
 
 def uu_decode(input, errors='strict'):
-    assert errors == 'strict'
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
     infile = BytesIO(input)
     outfile = BytesIO()
     readline = infile.readline
diff --git a/Lib/encodings/zlib_codec.py b/Lib/encodings/zlib_codec.py
index 95908a4b4a13a1..807d8b41ce40b2 100644
--- a/Lib/encodings/zlib_codec.py
+++ b/Lib/encodings/zlib_codec.py
@@ -8,14 +8,20 @@
 import codecs
 import zlib # this codec needs the optional zlib module !
 
+### Codec Helpers
+
+def _assert_strict(errors):
+    if errors != 'strict':
+        raise ValueError(f'Unsupported error handling mode: "{errors}" - must be "strict"')
+
 ### Codec APIs
 
 def zlib_encode(input, errors='strict'):
-    assert errors == 'strict'
+    _assert_strict(errors)
     return (zlib.compress(input), len(input))
 
 def zlib_decode(input, errors='strict'):
-    assert errors == 'strict'
+    _assert_strict(errors)
     return (zlib.decompress(input), len(input))
 
 class Codec(codecs.Codec):
@@ -26,7 +32,7 @@ def decode(self, input, errors='strict'):
 
 class IncrementalEncoder(codecs.IncrementalEncoder):
     def __init__(self, errors='strict'):
-        assert errors == 'strict'
+        _assert_strict(errors)
         self.errors = errors
         self.compressobj = zlib.compressobj()
 
@@ -42,7 +48,7 @@ def reset(self):
 
 class IncrementalDecoder(codecs.IncrementalDecoder):
     def __init__(self, errors='strict'):
-        assert errors == 'strict'
+        _assert_strict(errors)
         self.errors = errors
         self.decompressobj = zlib.decompressobj()
 
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index d8666f7290e72e..9b64888b082df8 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -3128,6 +3128,18 @@ def test_uu_invalid(self):
         # Missing "begin" line
         self.assertRaises(ValueError, codecs.decode, b"", "uu-codec")
 
+    def test_invalid_error_input(self):
+        # decoders/encoders require errors == 'strict'
+
+        for encoding in bytes_transform_encodings:
+            with self.subTest(encoding=encoding):
+                encoder = codecs.getencoder(encoding)
+                decoder = codecs.getdecoder(encoding)
+
+                self.assertRaises(ValueError, encoder, 'in', errors='notstrict')
+                self.assertRaises(ValueError, decoder, 'in', errors='notstrict')
+
+
 
 # The codec system tries to add notes to exceptions in order to ensure
 # the error mentions the operation being performed and the codec involved.
diff --git a/Misc/NEWS.d/next/Library/2025-07-13-07-54-49.gh-issue-62040.jryEkb.rst b/Misc/NEWS.d/next/Library/2025-07-13-07-54-49.gh-issue-62040.jryEkb.rst
new file mode 100644
index 00000000000000..19fe8636b2d3f0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-07-13-07-54-49.gh-issue-62040.jryEkb.rst
@@ -0,0 +1,4 @@
+The ``base64_codec``, ``uu_codec``, ``quopri_codec``, ``hex_codec``,
+``zlib_codec`` and ``bz2_codec`` now raise a :exc:`ValueError` when their
+decoder/encoder is provided an *errors* parameter that is not equal to
+``'strict'``.