diff --git a/partiql-planner/src/main/kotlin/org/partiql/planner/internal/typer/PlanTyper.kt b/partiql-planner/src/main/kotlin/org/partiql/planner/internal/typer/PlanTyper.kt
index 6281efad4d..47b727522d 100644
--- a/partiql-planner/src/main/kotlin/org/partiql/planner/internal/typer/PlanTyper.kt
+++ b/partiql-planner/src/main/kotlin/org/partiql/planner/internal/typer/PlanTyper.kt
@@ -183,7 +183,7 @@ internal class PlanTyper(
                     else -> type
                 }
             }.let {
-                unionOf(it.toSet())
+                unionOf(it.toSet()).flatten()
             }
 
             // rewrite