diff --git a/RNS/Cryptography/HKDF.py b/RNS/Cryptography/HKDF.py
index 111123a..0491dda 100644
--- a/RNS/Cryptography/HKDF.py
+++ b/RNS/Cryptography/HKDF.py
@@ -33,7 +33,7 @@ def hkdf(length=None, derive_from=None, salt=None, context=None):
     if length == None or length < 1:
         raise ValueError("Invalid output key length")
 
-    if derive_from == "None" or derive_from == "":
+    if derive_from == None or derive_from == "":
         raise ValueError("Cannot derive key from empty input material")
 
     if salt == None or len(salt) == 0:
@@ -54,4 +54,4 @@ def hkdf(length=None, derive_from=None, salt=None, context=None):
         block = hmac_sha256(pseudorandom_key, block + context + bytes([i + 1]))
         derived += block
 
-    return derived[:length]
\ No newline at end of file
+    return derived[:length]