diff --git a/src/embit/bip39.py b/src/embit/bip39.py
index 418f8b3..a6a8f85 100644
--- a/src/embit/bip39.py
+++ b/src/embit/bip39.py
@@ -11,7 +11,7 @@ def mnemonic_to_bytes(mnemonic: str, ignore_checksum: bool = False, wordlist=WOR
     # this function is copied from Jimmy Song's HDPrivateKey.from_mnemonic() method
 
     words = mnemonic.strip().split()
-    if len(words) % 3 != 0 or len(words) < 12:
+    if len(words) % 3 != 0 or not 12 <= len(words) <= 24:
         raise ValueError("Invalid recovery phrase")
 
     binary_seed = bytearray()
@@ -97,7 +97,7 @@ def _extract_index(bits, b, n):
 
 
 def mnemonic_from_bytes(entropy, wordlist=WORDLIST):
-    if len(entropy) % 4 != 0:
+    if len(entropy) % 4 != 0 or not 16 <= len(entropy) <= 32:
         raise ValueError("Byte array should be multiple of 4 long (16, 20, ..., 32)")
     total_bits = len(entropy) * 8
     checksum_bits = total_bits // 32
diff --git a/tests/tests/test_bip39.py b/tests/tests/test_bip39.py
index 685cb4e..f3b580d 100644
--- a/tests/tests/test_bip39.py
+++ b/tests/tests/test_bip39.py
@@ -173,8 +173,24 @@ def test_bip39(self):
             self.assertEqual(act_xkey.to_base58(), xprv)
 
     def test_invalid_length(self):
-        words = "panel trumpet seek bridge income piano history car flower aim loan accident embark canoe"
-        self.assertFalse(mnemonic_is_valid(words))
+        invalid_length = [
+            # not divisible by 3, too short, too long
+            "panel trumpet seek bridge income piano history car flower aim loan accident embark canoe",
+            "zoo " * 8 + "zebra",
+            "zoo " * 26 + "valley",
+        ]
+        for words in invalid_length:
+            self.assertFalse(mnemonic_is_valid(words))
+            self.assertRaises(ValueError, mnemonic_to_bytes, words)
+
+        invalid_length = [
+            # not divisible by 4, too short, too long
+            b"\x00" * 19,
+            b"\x00" * 12,
+            b"\x00" * 36,
+        ]
+        for entropy in invalid_length:
+            self.assertRaises(ValueError, mnemonic_from_bytes, entropy)
 
     def test_invalid_word(self):
         words = "fljsafk minute glow ride mask ceiling old limb rookie discover cotton biology"