diff --git a/scala/src/main/scala/Union.scala b/scala/src/main/scala/Union.scala
index f09037b..5b10f31 100644
--- a/scala/src/main/scala/Union.scala
+++ b/scala/src/main/scala/Union.scala
@@ -121,6 +121,7 @@ object Union {
         )
         // Invalid candidates are those where the left values are not existing
         .filter(col(leftTS).isNotNull)
+        .filter(col(rightTS).isNotNull)
         .drop(DF_INDEX_COLUMN)
         .drop(COMBINED_TS_COLUMN)