diff --git a/plasma_framework/python_tests/plasma_core/utils/utils.py b/plasma_framework/python_tests/plasma_core/utils/utils.py
index 7208cf492..525331584 100644
--- a/plasma_framework/python_tests/plasma_core/utils/utils.py
+++ b/plasma_framework/python_tests/plasma_core/utils/utils.py
@@ -20,6 +20,7 @@ def normalize_key(key):
 
 
 def hex_to_binary(h):
+    assert isinstance(h, str)
     if h[:2] == '0x':
         h = h[2:]
     return bytes.fromhex(h)