diff --git a/src/backend/utils/adt/datetime.c b/src/backend/utils/adt/datetime.c
index 2a5dddc43f..178b3f4780 100644
--- a/src/backend/utils/adt/datetime.c
+++ b/src/backend/utils/adt/datetime.c
@@ -3578,6 +3578,13 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 						break;
 
 					case AGO:
+
+						/*
+						 * "ago" is only allowed to appear at the end of the
+						 * interval.
+						 */
+						if (i != nf - 1)
+							return DTERR_BAD_FORMAT;
 						is_before = true;
 						type = uval;
 						break;
diff --git a/src/test/regress/expected/interval.out b/src/test/regress/expected/interval.out
index 28b71d9681..01d43b5868 100644
--- a/src/test/regress/expected/interval.out
+++ b/src/test/regress/expected/interval.out
@@ -1787,3 +1787,12 @@ SELECT extract(epoch from interval '1000000000 days');
  86400000000000.000000
 (1 row)
 
+-- "ago" can only appear once at the end of an interval.
+SELECT INTERVAL '42 days 2 seconds ago ago';
+ERROR:  invalid input syntax for type interval: "42 days 2 seconds ago ago"
+LINE 1: SELECT INTERVAL '42 days 2 seconds ago ago';
+                        ^
+SELECT INTERVAL '2 minutes ago 5 days';
+ERROR:  invalid input syntax for type interval: "2 minutes ago 5 days"
+LINE 1: SELECT INTERVAL '2 minutes ago 5 days';
+                        ^
diff --git a/src/test/regress/sql/interval.sql b/src/test/regress/sql/interval.sql
index 56feda1a3d..fb1ef30490 100644
--- a/src/test/regress/sql/interval.sql
+++ b/src/test/regress/sql/interval.sql
@@ -582,3 +582,7 @@ SELECT f1,
 
 -- internal overflow test case
 SELECT extract(epoch from interval '1000000000 days');
+
+-- "ago" can only appear once at the end of an interval.
+SELECT INTERVAL '42 days 2 seconds ago ago';
+SELECT INTERVAL '2 minutes ago 5 days';