diff --git a/common/lib/xmodule/xmodule/vertical_block.py b/common/lib/xmodule/xmodule/vertical_block.py
index ab3f7a9800..b812f0b844 100644
--- a/common/lib/xmodule/xmodule/vertical_block.py
+++ b/common/lib/xmodule/xmodule/vertical_block.py
@@ -78,7 +78,7 @@ class VerticalBlock(SequenceFields, XModuleFields, StudioEditableBlock, XmlParse
         # pylint: disable=no-member
         for child in child_blocks:
             child_block_context = copy(child_context)
-            if child in child_blocks_to_complete_on_view:
+            if child in list(child_blocks_to_complete_on_view):
                 child_block_context['wrap_xblock_data'] = {
                     'mark-completed-on-view-after-delay': complete_on_view_delay
                 }