diff --git a/themis/importexport/utils.py b/themis/importexport/utils.py
index 929b5e5..db9e7ba 100644
--- a/themis/importexport/utils.py
+++ b/themis/importexport/utils.py
@@ -128,6 +128,7 @@ def convert(value, attr=None, context=None):
         else:
             value = value.to_object.id
     elif isinstance(value, list) and value:
+        value = value[:]
         if isinstance(value[0], z3c.relationfield.relation.RelationValue):
             for i, v in enumerate(value):
                 if v.to_object is None: