diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index e7d6ff1b687..349a912427a 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -2233,9 +2233,18 @@ compute_semijoin_info(PlannerInfo *root, SpecialJoinInfo *sjinfo, List *clause)
 		if (!(all_btree || all_hash))
 			return;
 
+		/*
+		 * Ensure the RHS expression exposes the join's input collation (its
+		 * type should be OK already); see comments for
+		 * canonicalize_ec_expression.
+		 */
+		right_expr = (Node *) canonicalize_ec_expression((Expr *) copyObject(right_expr),
+														 exprType(right_expr),
+														 op->inputcollid);
+
 		/* so far so good, keep building lists */
 		semi_operators = lappend_oid(semi_operators, opno);
-		semi_rhs_exprs = lappend(semi_rhs_exprs, copyObject(right_expr));
+		semi_rhs_exprs = lappend(semi_rhs_exprs, right_expr);
 	}
 
 	/* Punt if we didn't find at least one column to unique-ify */
diff --git a/src/test/regress/expected/collate.icu.utf8.out b/src/test/regress/expected/collate.icu.utf8.out
index 0fd5c1a2cb0..8b294c1b73c 100644
--- a/src/test/regress/expected/collate.icu.utf8.out
+++ b/src/test/regress/expected/collate.icu.utf8.out
@@ -2104,6 +2104,60 @@ SELECT string_to_array('ABCDEFGHI' COLLATE case_insensitive, NULL, 'b');
  {A,NULL,C,D,E,F,G,H,I}
 (1 row)
 
+-- Unique-ifying a semijoin's RHS must use the join's collation.  test3cs
+-- holds both 'abc' and 'ABC', so test1ci's 'abc' must come out once.
+BEGIN;
+SET LOCAL enable_seqscan TO off;
+SET LOCAL enable_material TO off;
+SET LOCAL enable_hashjoin TO off;
+SET LOCAL enable_mergejoin TO off;
+SET LOCAL enable_hashagg TO off;
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+                                 QUERY PLAN                                 
+----------------------------------------------------------------------------
+ Aggregate
+   ->  Nested Loop
+         ->  Unique
+               ->  Sort
+                     Sort Key: ((test3cs.x)::text) COLLATE case_insensitive
+                     ->  Index Only Scan using test3cs_x_idx on test3cs
+         ->  Index Only Scan using test1ci_x_idx on test1ci
+               Index Cond: (x = (test3cs.x)::text)
+(8 rows)
+
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+ count 
+-------
+     3
+(1 row)
+
+SET LOCAL enable_hashagg TO on;
+SET LOCAL enable_sort TO off;
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Aggregate
+   ->  Nested Loop
+         ->  HashAggregate
+               Group Key: (test3cs.x)::text
+               ->  Index Only Scan using test3cs_x_idx on test3cs
+         ->  Index Only Scan using test1ci_x_idx on test1ci
+               Index Cond: (x = (test3cs.x)::text)
+(7 rows)
+
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+ count 
+-------
+     3
+(1 row)
+
+ROLLBACK;
 -- These queries should be able to use the index on test1ci.x:
 SET enable_seqscan = off;
 SET enable_indexonlyscan = off;
diff --git a/src/test/regress/sql/collate.icu.utf8.sql b/src/test/regress/sql/collate.icu.utf8.sql
index b091f05c113..2b38c640428 100644
--- a/src/test/regress/sql/collate.icu.utf8.sql
+++ b/src/test/regress/sql/collate.icu.utf8.sql
@@ -747,6 +747,33 @@ CREATE UNIQUE INDEX ON test3ci (x);  -- error
 SELECT string_to_array('ABC,DEF,GHI' COLLATE case_insensitive, ',', 'abc');
 SELECT string_to_array('ABCDEFGHI' COLLATE case_insensitive, NULL, 'b');
 
+-- Unique-ifying a semijoin's RHS must use the join's collation.  test3cs
+-- holds both 'abc' and 'ABC', so test1ci's 'abc' must come out once.
+BEGIN;
+
+SET LOCAL enable_seqscan TO off;
+SET LOCAL enable_material TO off;
+SET LOCAL enable_hashjoin TO off;
+SET LOCAL enable_mergejoin TO off;
+SET LOCAL enable_hashagg TO off;
+
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+
+SET LOCAL enable_hashagg TO on;
+SET LOCAL enable_sort TO off;
+
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+SELECT count(*) FROM test1ci
+WHERE x COLLATE case_insensitive IN (SELECT x FROM test3cs);
+
+ROLLBACK;
+
 -- These queries should be able to use the index on test1ci.x:
 SET enable_seqscan = off;
 SET enable_indexonlyscan = off;