diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
index bfa1ea01024..9b0bde773f5 100644
--- a/src/backend/commands/matview.c
+++ b/src/backend/commands/matview.c
@@ -792,9 +792,12 @@ refresh_by_match_merge(Oid matviewOid, Oid tempOid, Oid relowner,
 	 *
 	 * ExecRefreshMatView() checks that after taking the exclusive lock on the
 	 * matview. So at least one unique index is guaranteed to exist here
-	 * because the lock is still being held; so an Assert seems sufficient.
+	 * because the lock is still being held.  (One known exception is if a
+	 * function called as part of refreshing the matview drops the index.
+	 * That's a pretty silly thing to do.)
 	 */
-	Assert(foundUniqueIndex);
+	if (!foundUniqueIndex)
+		elog(ERROR, "could not find suitable unique index on materialized view");
 
 	appendStringInfoString(&querybuf,
 						   " AND newdata.* OPERATOR(pg_catalog.*=) mv.*) "
diff --git a/src/test/regress/expected/matview.out b/src/test/regress/expected/matview.out
index 93d6e23b4a0..18c5b866a5a 100644
--- a/src/test/regress/expected/matview.out
+++ b/src/test/regress/expected/matview.out
@@ -572,6 +572,22 @@ REFRESH MATERIALIZED VIEW mvtest_mv_foo;
 REFRESH MATERIALIZED VIEW CONCURRENTLY mvtest_mv_foo;
 DROP OWNED BY regress_user_mvtest CASCADE;
 DROP ROLE regress_user_mvtest;
+-- Concurrent refresh requires a unique index on the materialized
+-- view. Test what happens if it's dropped during the refresh.
+CREATE OR REPLACE FUNCTION mvtest_drop_the_index()
+  RETURNS bool AS $$
+BEGIN
+  EXECUTE 'DROP INDEX IF EXISTS mvtest_drop_idx';
+  RETURN true;
+END;
+$$ LANGUAGE plpgsql;
+CREATE MATERIALIZED VIEW drop_idx_matview AS
+  SELECT 1 as i WHERE mvtest_drop_the_index();
+NOTICE:  index "mvtest_drop_idx" does not exist, skipping
+CREATE UNIQUE INDEX mvtest_drop_idx ON drop_idx_matview (i);
+REFRESH MATERIALIZED VIEW CONCURRENTLY drop_idx_matview;
+ERROR:  could not find suitable unique index on materialized view
+DROP MATERIALIZED VIEW drop_idx_matview; -- clean up
 -- make sure that create WITH NO DATA works via SPI
 BEGIN;
 CREATE FUNCTION mvtest_func()
diff --git a/src/test/regress/sql/matview.sql b/src/test/regress/sql/matview.sql
index 1d690254641..385e48e9e78 100644
--- a/src/test/regress/sql/matview.sql
+++ b/src/test/regress/sql/matview.sql
@@ -231,6 +231,23 @@ REFRESH MATERIALIZED VIEW CONCURRENTLY mvtest_mv_foo;
 DROP OWNED BY regress_user_mvtest CASCADE;
 DROP ROLE regress_user_mvtest;
 
+-- Concurrent refresh requires a unique index on the materialized
+-- view. Test what happens if it's dropped during the refresh.
+CREATE OR REPLACE FUNCTION mvtest_drop_the_index()
+  RETURNS bool AS $$
+BEGIN
+  EXECUTE 'DROP INDEX IF EXISTS mvtest_drop_idx';
+  RETURN true;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE MATERIALIZED VIEW drop_idx_matview AS
+  SELECT 1 as i WHERE mvtest_drop_the_index();
+
+CREATE UNIQUE INDEX mvtest_drop_idx ON drop_idx_matview (i);
+REFRESH MATERIALIZED VIEW CONCURRENTLY drop_idx_matview;
+DROP MATERIALIZED VIEW drop_idx_matview; -- clean up
+
 -- make sure that create WITH NO DATA works via SPI
 BEGIN;
 CREATE FUNCTION mvtest_func()