diff --git a/index/field/multi_field_union_iterator.go b/index/field/multi_field_union_iterator.go
index d75e932..950b7e7 100644
--- a/index/field/multi_field_union_iterator.go
+++ b/index/field/multi_field_union_iterator.go
@@ -44,7 +44,7 @@ func (it *MultiFieldUnionIterator) DocID() int32 { return it.unionIter.DocID() }
 // value array is valid.
 func (it *MultiFieldUnionIterator) Values() ([]field.ValueUnion, []bool) {
 	for i := 0; i < len(it.iters); i++ {
-		if it.unionIter.IsDoneAt(i) || it.unionIter.DocID() != it.iters[i].DocID() {
+		if !it.unionIter.IsValidAt(i) {
 			it.hasVals[i] = false
 		} else {
 			it.hasVals[i] = true
diff --git a/index/in_any_doc_id_set_iterator.go b/index/in_any_doc_id_set_iterator.go
index 9e61ab3..3aca204 100644
--- a/index/in_any_doc_id_set_iterator.go
+++ b/index/in_any_doc_id_set_iterator.go
@@ -87,9 +87,10 @@ func (it *InAnyDocIDSetIterator) DocID() int32 {
 	return it.docIDs[it.minDocIDIdx]
 }
 
-// IsDoneAt returns whether the iterator at a given index is done.
-func (it *InAnyDocIDSetIterator) IsDoneAt(idx int) bool {
-	return it.docIDs[idx] == invalidDocID
+// IsValidAt returns true if the base iterator at a given index has the same doc ID
+// as the current doc ID of the `InAnyDocIDSetIterator`.
+func (it *InAnyDocIDSetIterator) IsValidAt(idx int) bool {
+	return it.docIDs[idx] != invalidDocID && it.docIDs[idx] == it.DocID()
 }
 
 // Err returns any errors encountered.