diff --git a/src/test/regress/expected/interval.out b/src/test/regress/expected/interval.out
index 8a4eb27590..ed16ada224 100644
--- a/src/test/regress/expected/interval.out
+++ b/src/test/regress/expected/interval.out
@@ -212,3 +212,12 @@ SELECT '' AS ten, INTERVAL_TBL.*;
      | @ 5 mons 12 hours
 (10 rows)
 
+-- test avg(interval), which is somewhat fragile since people have been
+-- known to change the allowed input syntax for type interval without
+-- updating pg_aggregate.agginitval
+select avg(f1) from interval_tbl;
+                       avg                       
+-------------------------------------------------
+ @ 4 years 1 mon 10 days 4 hours 18 mins 23 secs
+(1 row)
+
diff --git a/src/test/regress/sql/interval.sql b/src/test/regress/sql/interval.sql
index 945eb08d60..aa14cd3b09 100644
--- a/src/test/regress/sql/interval.sql
+++ b/src/test/regress/sql/interval.sql
@@ -60,3 +60,9 @@ SELECT '' AS fortyfive, r1.*, r2.*
 SET DATESTYLE = 'postgres';
 
 SELECT '' AS ten, INTERVAL_TBL.*;
+
+-- test avg(interval), which is somewhat fragile since people have been
+-- known to change the allowed input syntax for type interval without
+-- updating pg_aggregate.agginitval
+
+select avg(f1) from interval_tbl;