diff --git a/adafruit_rsa/transform.py b/adafruit_rsa/transform.py
index 922a1ab..a64f523 100755
--- a/adafruit_rsa/transform.py
+++ b/adafruit_rsa/transform.py
@@ -178,7 +178,7 @@ def int2bytes(number, fill_size=None, chunk_size=None, overflow=False):
         raise ValueError("You can either fill or pad chunks, but not both")
 
     # Ensure these are integers.
-    assert number & 1 == 0, "Number must be an unsigned integer, not a float."
+    assert isinstance(number, int), "Number must be an unsigned integer, not a float."
 
     raw_bytes = b""
 
@@ -202,7 +202,7 @@ def int2bytes(number, fill_size=None, chunk_size=None, overflow=False):
             raise OverflowError(
                 "Need %d bytes for number, but fill size is %d" % (length, fill_size)
             )
-        raw_bytes = "% {}s".format(fill_size).encode() % raw_bytes
+        raw_bytes = (b"\x00" * (fill_size - len(raw_bytes))) + raw_bytes
     elif chunk_size and chunk_size > 0:
         remainder = length % chunk_size
         if remainder: