diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 09fabf8943b7917287d40dcfabda456702e2b119..c47594a39eb06e511fa793af11c6eb278e3bbd52 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -101,7 +101,7 @@ def indexes(table_name)
       def index_exists?(table_name, column_name, options = {})
         column_names = Array(column_name).map(&:to_s)
         checks = []
-        checks << lambda { |i| i.columns == column_names }
+        checks << lambda { |i| Array(i.columns) == column_names }
         checks << lambda { |i| i.unique } if options[:unique]
         checks << lambda { |i| i.name == options[:name].to_s } if options[:name]
 
diff --git a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
index b1acdbbbbeeb211dc502810bdeca192690caec14..2a9dc1e328647fa3dc67b1eabf5ba97a2fcc8057 100644
--- a/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb
@@ -253,9 +253,11 @@ def test_partial_index
 
       def test_expression_index
         with_example_table do
-          @connection.add_index "ex", "mod(id, 10), abs(number)", name: "expression"
+          expr = "mod(id, 10), abs(number)"
+          @connection.add_index "ex", expr, name: "expression"
           index = @connection.indexes("ex").find { |idx| idx.name == "expression" }
-          assert_equal "mod(id, 10), abs(number)", index.columns
+          assert_equal expr, index.columns
+          assert_equal true, @connection.index_exists?("ex", expr, name: "expression")
         end
       end