diff --git a/ether/asm.py b/ether/asm.py
index f4319011..a4ce65a8 100644
--- a/ether/asm.py
+++ b/ether/asm.py
@@ -153,6 +153,6 @@ def assemble(disassembly):
 
         if 'argument' in instruction:
 
-            bytecode += bytes(instruction['argument'], 'ascii')
+            bytecode += codecs.decode(instruction['argument'], 'hex_codec')
 
     return bytecode