diff --git a/pybtc/hdwallet.py b/pybtc/hdwallet.py
index 54d0383..e356189 100644
--- a/pybtc/hdwallet.py
+++ b/pybtc/hdwallet.py
@@ -11,9 +11,11 @@ from .constants import *
 #
 
 def create_passphrase(bits=256, language='english'):
-    entropy = os.urandom(bits // 8)
-    mnemonic = create_mnemonic(entropy, language)
-    return ' '.join(mnemonic[::-1])
+    if bits in [128, 160, 192, 224, 256]:
+        entropy = os.urandom(bits // 8)
+        mnemonic = create_mnemonic(entropy, language)
+        return ' '.join(mnemonic[::-1])
+    raise ValueError('Strength should be one of the following [128, 160, 192, 224, 256], but it is not (%d).' % bits)
 
 
 def create_mnemonic(entropy, language='english'):
@@ -57,22 +59,24 @@ def add_checksum(data):
 
 def mnemonic2bytes(passphrase, language):
     mnemonic = passphrase.split()
-    wordlist = create_wordlist(language)
-    codes = dict()
-    for code, word in enumerate(wordlist):
-        codes[word] = code
-    word_count = len(mnemonic)
-    entropy_int = None
-    bit_size = word_count * 11
-    chk_sum_bit_len = word_count * 11 % 32
-    for word in mnemonic:
-        entropy_int = (entropy_int << 11) | codes[word] if entropy_int else codes[word]
-    chk_sum = entropy_int & (2 ** chk_sum_bit_len - 1)
-    entropy_int = entropy_int >> chk_sum_bit_len
-    entropy = entropy_int.to_bytes((bit_size - chk_sum_bit_len) // 8, byteorder="big")
-    ent_hash = hashlib.sha256(entropy).hexdigest()
-    fb = unhexlify(ent_hash)[0]
-    assert (fb >> (8 - chk_sum_bit_len)) & chk_sum
-    return entropy
+    if len(mnemonic) in [12, 15, 18, 21, 24]:
+        wordlist = create_wordlist(language)
+        codes = dict()
+        for code, word in enumerate(wordlist):
+            codes[word] = code
+        word_count = len(mnemonic)
+        entropy_int = None
+        bit_size = word_count * 11
+        chk_sum_bit_len = word_count * 11 % 32
+        for word in mnemonic:
+            entropy_int = (entropy_int << 11) | codes[word] if entropy_int else codes[word]
+        chk_sum = entropy_int & (2 ** chk_sum_bit_len - 1)
+        entropy_int = entropy_int >> chk_sum_bit_len
+        entropy = entropy_int.to_bytes((bit_size - chk_sum_bit_len) // 8, byteorder="big")
+        ent_hash = hashlib.sha256(entropy).hexdigest()
+        fb = unhexlify(ent_hash)[0]
+        assert (fb >> (8 - chk_sum_bit_len)) & chk_sum
+        return entropy
+    raise ValueError('Number of words must be one of the following: [12, 15, 18, 21, 24], but it is not (%d).' % len(mnemonic))