diff --git a/src/backend/access/brin/brin_tuple.c b/src/backend/access/brin/brin_tuple.c
index 8d03e609a3..32ffd9f9fc 100644
--- a/src/backend/access/brin/brin_tuple.c
+++ b/src/backend/access/brin/brin_tuple.c
@@ -220,10 +220,12 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 
 				/*
 				 * If the BRIN summary and indexed attribute use the same data
-				 * type, we can use the same compression method. Otherwise we
-				 * have to use the default method.
+				 * type and it has a valid compression method, we can use the
+				 * same compression method. Otherwise we have to use the
+				 * default method.
 				 */
-				if (att->atttypid == atttype->type_id)
+				if (att->atttypid == atttype->type_id &&
+					CompressionMethodIsValid(att->attcompression))
 					compression = att->attcompression;
 				else
 					compression = GetDefaultToastCompression();
diff --git a/src/backend/access/common/indextuple.c b/src/backend/access/common/indextuple.c
index 1f6b7b77d4..ae932691af 100644
--- a/src/backend/access/common/indextuple.c
+++ b/src/backend/access/common/indextuple.c
@@ -103,8 +103,19 @@ index_form_tuple(TupleDesc tupleDescriptor,
 			(att->attstorage == TYPSTORAGE_EXTENDED ||
 			 att->attstorage == TYPSTORAGE_MAIN))
 		{
-			Datum		cvalue = toast_compress_datum(untoasted_values[i],
-													  att->attcompression);
+			Datum	cvalue;
+			char	compression = att->attcompression;
+
+			/*
+			 * If the compression method is not valid, use the default. We
+			 * don't expect this to happen for regular index columns, which
+			 * inherit the setting from the corresponding table column, but
+			 * we do expect it to happen whenever an expression is indexed.
+			 */
+			if (!CompressionMethodIsValid(compression))
+				compression = GetDefaultToastCompression();
+
+			cvalue = toast_compress_datum(untoasted_values[i], compression);
 
 			if (DatumGetPointer(cvalue) != NULL)
 			{
diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index af30840c04..a628b3281c 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -30,6 +30,7 @@
 #include "access/relscan.h"
 #include "access/sysattr.h"
 #include "access/tableam.h"
+#include "access/toast_compression.h"
 #include "access/transam.h"
 #include "access/visibilitymap.h"
 #include "access/xact.h"
@@ -379,6 +380,15 @@ ConstructTupleDescriptor(Relation heapRelation,
 			to->attalign = typeTup->typalign;
 			to->atttypmod = exprTypmod(indexkey);
 
+			/*
+			 * For expression columns, set attcompression invalid, since
+			 * there's no table column from which to copy the value. Whenever
+			 * we actually need to compress a value, we'll use whatever the
+			 * current value of default_compression_method is at that point
+			 * in time.
+			 */
+			to->attcompression = InvalidCompressionMethod;
+
 			ReleaseSysCache(tuple);
 
 			/*
diff --git a/src/test/regress/expected/compression.out b/src/test/regress/expected/compression.out
index 566a1877ea..61e97cb33c 100644
--- a/src/test/regress/expected/compression.out
+++ b/src/test/regress/expected/compression.out
@@ -313,6 +313,12 @@ SELECT pg_column_compression(f1) FROM cmdata;
  lz4
 (2 rows)
 
+-- test expression index
+DROP TABLE cmdata2;
+CREATE TABLE cmdata2 (f1 TEXT COMPRESSION pglz, f2 TEXT COMPRESSION lz4);
+CREATE UNIQUE INDEX idx1 ON cmdata2 ((f1 || f2));
+INSERT INTO cmdata2 VALUES((SELECT array_agg(md5(g::TEXT))::TEXT FROM
+generate_series(1, 50) g), VERSION());
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
diff --git a/src/test/regress/expected/compression_1.out b/src/test/regress/expected/compression_1.out
index 3990933415..d03d6255ae 100644
--- a/src/test/regress/expected/compression_1.out
+++ b/src/test/regress/expected/compression_1.out
@@ -309,6 +309,19 @@ SELECT pg_column_compression(f1) FROM cmdata;
  pglz
 (2 rows)
 
+-- test expression index
+DROP TABLE cmdata2;
+CREATE TABLE cmdata2 (f1 TEXT COMPRESSION pglz, f2 TEXT COMPRESSION lz4);
+ERROR:  unsupported LZ4 compression method
+DETAIL:  This functionality requires the server to be built with lz4 support.
+HINT:  You need to rebuild PostgreSQL using --with-lz4.
+CREATE UNIQUE INDEX idx1 ON cmdata2 ((f1 || f2));
+ERROR:  relation "cmdata2" does not exist
+INSERT INTO cmdata2 VALUES((SELECT array_agg(md5(g::TEXT))::TEXT FROM
+generate_series(1, 50) g), VERSION());
+ERROR:  relation "cmdata2" does not exist
+LINE 1: INSERT INTO cmdata2 VALUES((SELECT array_agg(md5(g::TEXT))::...
+                    ^
 -- check data is ok
 SELECT length(f1) FROM cmdata;
  length 
diff --git a/src/test/regress/sql/compression.sql b/src/test/regress/sql/compression.sql
index 5e178be04a..76d1776d83 100644
--- a/src/test/regress/sql/compression.sql
+++ b/src/test/regress/sql/compression.sql
@@ -130,6 +130,13 @@ SELECT pg_column_compression(f1) FROM cmdata;
 VACUUM FULL cmdata;
 SELECT pg_column_compression(f1) FROM cmdata;
 
+-- test expression index
+DROP TABLE cmdata2;
+CREATE TABLE cmdata2 (f1 TEXT COMPRESSION pglz, f2 TEXT COMPRESSION lz4);
+CREATE UNIQUE INDEX idx1 ON cmdata2 ((f1 || f2));
+INSERT INTO cmdata2 VALUES((SELECT array_agg(md5(g::TEXT))::TEXT FROM
+generate_series(1, 50) g), VERSION());
+
 -- check data is ok
 SELECT length(f1) FROM cmdata;
 SELECT length(f1) FROM cmdata1;