diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 4160d2d6e24..f2b58ebfe1e 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -2265,6 +2265,18 @@ estimate_array_length(PlannerInfo *root, Node *arrayexpr)
 		AttStatsSlot sslot;
 		double		nelem = 0;
 
+		/*
+		 * Skip calling examine_variable for Var with varno 0, which has no
+		 * valid relation entry and would error in find_base_rel.  Such a Var
+		 * can appear when a nested set operation's output type doesn't match
+		 * the parent's expected type, because recurse_set_operations builds a
+		 * projection target list using generate_setop_tlist with varno 0, and
+		 * if the required type coercion involves an ArrayCoerceExpr, we can
+		 * be called on that Var.
+		 */
+		if (IsA(arrayexpr, Var) && ((Var *) arrayexpr)->varno == 0)
+			return 10;			/* default guess, should match scalararraysel */
+
 		examine_variable(root, arrayexpr, 0, &vardata);
 		if (HeapTupleIsValid(vardata.statsTuple))
 		{
diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index 709c85f2294..3a49b354058 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -1661,3 +1661,20 @@ on true limit 1;
                ->  Result
 (6 rows)
 
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];
+                 QUERY PLAN                  
+---------------------------------------------
+ Append
+   ->  Result
+         Output: (NULL::integer[])
+         ->  Append
+               ->  Result
+                     Output: NULL::integer[]
+               ->  Result
+                     Output: NULL::integer[]
+   ->  Result
+         Output: NULL::bigint[]
+(10 rows)
+
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index d0c70fafbea..078c858429f 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -664,3 +664,7 @@ select * from tenk1 t1
 left join lateral
   (select t1.tenthous from tenk2 t2 union all (values(1)))
 on true limit 1;
+
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];