diff --git a/acme/acme/jose/json_util.py b/acme/acme/jose/json_util.py
index 0608d3906..51d55ebd9 100644
--- a/acme/acme/jose/json_util.py
+++ b/acme/acme/jose/json_util.py
@@ -326,7 +326,8 @@ def decode_b64jose(data, size=None, minimum=False):
 
     if size is not None and ((not minimum and len(decoded) != size)
                              or (minimum and len(decoded) < size)):
-        raise errors.DeserializationError()
+        raise errors.DeserializationError(
+            "Expected at least or exactly {0} bytes".format(size))
 
     return decoded