diff --git a/src/org/python/modules/_marshal.java b/src/org/python/modules/_marshal.java
--- a/src/org/python/modules/_marshal.java
+++ b/src/org/python/modules/_marshal.java
@@ -44,7 +44,6 @@
     private final static char TYPE_DICT = '{';
     private final static char TYPE_CODE = 'c';
     private final static char TYPE_UNICODE = 'u';
-    private final static char TYPE_UNKNOWN = '?';
     private final static char TYPE_SET = '<';
     private final static char TYPE_FROZENSET = '>';
     private final static int MAX_MARSHAL_STACK_DEPTH = 2000;
@@ -237,7 +236,7 @@
                 write_int(code.co_firstlineno);
                 write_object(Py.newString(new String(code.co_lnotab)), depth + 1);
             } else {
-                write_byte(TYPE_UNKNOWN);
+                throw Py.ValueError("unmarshallable object");
             }

             depth--;