diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,9 @@
 # Changelog
 
+- 0.3.0 (2025-06-21)
+  * The 'mac' function is now total, returning 'Nothing' when supplied
+    with an invalid-length one-time key.
+
 - 0.2.0 (2025-03-10)
   * Fix a bug in which small produced MAC's were not being padded to 128
     bits.
diff --git a/bench/Main.hs b/bench/Main.hs
--- a/bench/Main.hs
+++ b/bench/Main.hs
@@ -18,13 +18,23 @@
 msg = fromJust . B16.decode $
   "4c616469657320616e642047656e746c656d656e206f662074686520636c617373206f66202739393a204966204920636f756c64206f6666657220796f75206f6e6c79206f6e652074697020666f7220746865206675747572652c2073756e73637265656e20776f756c642062652069742e"
 
-key :: BS.ByteString
-key = fromJust . B16.decode $
-  "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f"
+key_small :: BS.ByteString
+key_small = fromJust . B16.decode $
+  "0000000000000000000000000000000000000000000000000000000000000003"
 
+key_mid :: BS.ByteString
+key_mid = fromJust . B16.decode $
+  "8888888888888888888888888888888888888888888888888888888888888883"
+
+key_big :: BS.ByteString
+key_big = fromJust . B16.decode $
+  "fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff3"
+
 suite :: Benchmark
 suite =
   bgroup "ppad-poly1305" [
-    bench "mac" $ nf (Poly1305.mac key) msg
+    bench "mac (small key)" $ nf (Poly1305.mac key_small) msg
+  , bench "mac (mid key)" $ nf (Poly1305.mac key_mid) msg
+  , bench "mac (big key)" $ nf (Poly1305.mac key_big) msg
   ]
 
diff --git a/lib/Crypto/MAC/Poly1305.hs b/lib/Crypto/MAC/Poly1305.hs
--- a/lib/Crypto/MAC/Poly1305.hs
+++ b/lib/Crypto/MAC/Poly1305.hs
@@ -64,20 +64,19 @@
 --   Per RFC8439: the key, which is essentially a /one-time/ key, should
 --   be unique, and MUST be unpredictable for each invocation.
 --
---   The key must be exactly 256 bits in length. Providing an invalid
---   key will cause the function to throw an ErrorCall exception.
+--   The key must be exactly 256 bits in length.
 --
 --   >>> mac "i'll never use this key again!!!" "a message needing authentication"
---   "O'\231Z\224\149\148\246\203[}\210\203\b\200\207"
+--   Just "O'\231Z\224\149\148\246\203[}\210\203\b\200\207"
 mac
   :: BS.ByteString -- ^ 256-bit one-time key
   -> BS.ByteString -- ^ arbitrary-length message
-  -> BS.ByteString -- ^ 128-bit message authentication code
+  -> Maybe BS.ByteString -- ^ 128-bit message authentication code
 mac key@(BI.PS _ _ kl) msg
-  | kl /= 32  = error "ppad-poly1305 (mac): invalid key"
+  | kl /= 32  = Nothing
   | otherwise =
       let (clamp . _roll -> r, _roll -> s) = BS.splitAt 16 key
-      in  _poly1305_loop r s msg
+      in  pure (_poly1305_loop r s msg)
 
 _poly1305_loop :: Integer -> Integer -> BS.ByteString -> BS.ByteString
 _poly1305_loop !r !s !msg =
diff --git a/ppad-poly1305.cabal b/ppad-poly1305.cabal
--- a/ppad-poly1305.cabal
+++ b/ppad-poly1305.cabal
@@ -1,6 +1,6 @@
 cabal-version:      3.0
 name:               ppad-poly1305
-version:            0.2.0
+version:            0.3.0
 synopsis:           A pure Poly1305 MAC
 license:            MIT
 license-file:       LICENSE
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -34,7 +34,7 @@
 
       Just e = B16.decode "a8061dc1305136c6c22b8baf0c0127a9"
 
-      o = Poly1305.mac key msg
+      Just o = Poly1305.mac key msg
   H.assertEqual mempty e o
 
 mac1 :: TestTree
@@ -45,7 +45,7 @@
         "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
       Just tag = B16.decode
         "00000000000000000000000000000000"
-      out = Poly1305.mac key msg
+      Just out = Poly1305.mac key msg
   H.assertEqual mempty tag out
 
 mac2 :: TestTree
@@ -56,7 +56,7 @@
         "416e79207375626d697373696f6e20746f20746865204945544620696e74656e6465642062792074686520436f6e7472696275746f7220666f72207075626c69636174696f6e20617320616c6c206f722070617274206f6620616e204945544620496e7465726e65742d4472616674206f722052464320616e6420616e792073746174656d656e74206d6164652077697468696e2074686520636f6e74657874206f6620616e204945544620616374697669747920697320636f6e7369646572656420616e20224945544620436f6e747269627574696f6e222e20537563682073746174656d656e747320696e636c756465206f72616c2073746174656d656e747320696e20494554462073657373696f6e732c2061732077656c6c206173207772697474656e20616e6420656c656374726f6e696320636f6d6d756e69636174696f6e73206d61646520617420616e792074696d65206f7220706c6163652c207768696368206172652061646472657373656420746f"
       Just tag = B16.decode
         "36e5f6b5c5e06070f0efca96227a863e"
-      out = Poly1305.mac key msg
+      Just out = Poly1305.mac key msg
   H.assertEqual mempty tag out
 
 mac3 :: TestTree
@@ -67,7 +67,7 @@
         "416e79207375626d697373696f6e20746f20746865204945544620696e74656e6465642062792074686520436f6e7472696275746f7220666f72207075626c69636174696f6e20617320616c6c206f722070617274206f6620616e204945544620496e7465726e65742d4472616674206f722052464320616e6420616e792073746174656d656e74206d6164652077697468696e2074686520636f6e74657874206f6620616e204945544620616374697669747920697320636f6e7369646572656420616e20224945544620436f6e747269627574696f6e222e20537563682073746174656d656e747320696e636c756465206f72616c2073746174656d656e747320696e20494554462073657373696f6e732c2061732077656c6c206173207772697474656e20616e6420656c656374726f6e696320636f6d6d756e69636174696f6e73206d61646520617420616e792074696d65206f7220706c6163652c207768696368206172652061646472657373656420746f"
       Just tag = B16.decode
         "f3477e7cd95417af89a6b8794c310cf0"
-      out = Poly1305.mac key msg
+      Just out = Poly1305.mac key msg
   H.assertEqual mempty tag out
 
 mac4 :: TestTree
@@ -78,7 +78,7 @@
         "2754776173206272696c6c69672c20616e642074686520736c6974687920746f7665730a446964206779726520616e642067696d626c6520696e2074686520776162653a0a416c6c206d696d737920776572652074686520626f726f676f7665732c0a416e6420746865206d6f6d65207261746873206f757467726162652e"
       Just tag = B16.decode
         "4541669a7eaaee61e708dc7cbcc5eb62"
-      out = Poly1305.mac key msg
+      Just out = Poly1305.mac key msg
   H.assertEqual mempty tag out
 
 mac5 :: TestTree