diff --git a/src/backend/parser/parse_collate.c b/src/backend/parser/parse_collate.c
index 0b77e3ea2b..8860679ccb 100644
--- a/src/backend/parser/parse_collate.c
+++ b/src/backend/parser/parse_collate.c
@@ -533,6 +533,30 @@ assign_collations_walker(Node *node, assign_collations_context *context)
 			collation = loccontext.collation;
 			strength = loccontext.strength;
 			location = loccontext.location;
+
+			/*
+			 * Throw error if the collation is indeterminate for a TargetEntry
+			 * that is a sort/group target.  We prefer to do this now, instead
+			 * of leaving the comparison functions to fail at runtime, because
+			 * we can give a syntax error pointer to help locate the problem.
+			 * There are some cases where there might not be a failure, for
+			 * example if the planner chooses to use hash aggregation instead
+			 * of sorting for grouping; but it seems better to predictably
+			 * throw an error.  (Compare transformSetOperationTree, which will
+			 * throw error for indeterminate collation of set-op columns,
+			 * even though the planner might be able to implement the set-op
+			 * without sorting.)
+			 */
+			if (strength == COLLATE_CONFLICT &&
+				((TargetEntry *) node)->ressortgroupref != 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_COLLATION_MISMATCH),
+						 errmsg("collation mismatch between implicit collations \"%s\" and \"%s\"",
+								get_collation_name(loccontext.collation),
+								get_collation_name(loccontext.collation2)),
+						 errhint("You can choose the collation by applying the COLLATE clause to one or both expressions."),
+						 parser_errposition(context->pstate,
+											loccontext.location2)));
 			break;
 		case T_RangeTblRef:
 		case T_JoinExpr:
diff --git a/src/test/regress/expected/collate.linux.utf8.out b/src/test/regress/expected/collate.linux.utf8.out
index f967998be6..0ed0b36da7 100644
--- a/src/test/regress/expected/collate.linux.utf8.out
+++ b/src/test/regress/expected/collate.linux.utf8.out
@@ -637,6 +637,11 @@ select x || y from collate_test10; -- ok, because || is not collation aware
  HIJHIJ
 (2 rows)
 
+select x, y from collate_test10 order by x || y; -- not so ok
+ERROR:  collation mismatch between implicit collations "en_US.utf8" and "tr_TR.utf8"
+LINE 1: select x, y from collate_test10 order by x || y;
+                                                      ^
+HINT:  You can choose the collation by applying the COLLATE clause to one or both expressions.
 -- collation mismatch between recursive and non-recursive term
 WITH RECURSIVE foo(x) AS
    (SELECT x FROM (VALUES('a' COLLATE "en_US"),('b')) t(x)
diff --git a/src/test/regress/expected/collate.out b/src/test/regress/expected/collate.out
index bd25955c5e..4536cf2101 100644
--- a/src/test/regress/expected/collate.out
+++ b/src/test/regress/expected/collate.out
@@ -443,6 +443,11 @@ select x || y from collate_test10; -- ok, because || is not collation aware
  HIJHIJ
 (2 rows)
 
+select x, y from collate_test10 order by x || y; -- not so ok
+ERROR:  collation mismatch between implicit collations "C" and "POSIX"
+LINE 1: select x, y from collate_test10 order by x || y;
+                                                      ^
+HINT:  You can choose the collation by applying the COLLATE clause to one or both expressions.
 -- collation mismatch between recursive and non-recursive term
 WITH RECURSIVE foo(x) AS
    (SELECT x FROM (VALUES('a' COLLATE "C"),('b')) t(x)
diff --git a/src/test/regress/sql/collate.linux.utf8.sql b/src/test/regress/sql/collate.linux.utf8.sql
index 62e66811b6..f5c5d802b2 100644
--- a/src/test/regress/sql/collate.linux.utf8.sql
+++ b/src/test/regress/sql/collate.linux.utf8.sql
@@ -193,6 +193,7 @@ CREATE TABLE test_u AS SELECT a, b FROM collate_test1 UNION ALL SELECT a, b FROM
 -- ideally this would be a parse-time error, but for now it must be run-time:
 select x < y from collate_test10; -- fail
 select x || y from collate_test10; -- ok, because || is not collation aware
+select x, y from collate_test10 order by x || y; -- not so ok
 
 -- collation mismatch between recursive and non-recursive term
 WITH RECURSIVE foo(x) AS
diff --git a/src/test/regress/sql/collate.sql b/src/test/regress/sql/collate.sql
index a3926a9e79..c7e26d8577 100644
--- a/src/test/regress/sql/collate.sql
+++ b/src/test/regress/sql/collate.sql
@@ -145,6 +145,7 @@ CREATE TABLE test_u AS SELECT a, b FROM collate_test1 UNION ALL SELECT a, b FROM
 -- ideally this would be a parse-time error, but for now it must be run-time:
 select x < y from collate_test10; -- fail
 select x || y from collate_test10; -- ok, because || is not collation aware
+select x, y from collate_test10 order by x || y; -- not so ok
 
 -- collation mismatch between recursive and non-recursive term
 WITH RECURSIVE foo(x) AS