diff --git a/lib/active_record/connection_adapters/sqlserver/database_statements.rb b/lib/active_record/connection_adapters/sqlserver/database_statements.rb
index 7a825101d..97e66c155 100644
--- a/lib/active_record/connection_adapters/sqlserver/database_statements.rb
+++ b/lib/active_record/connection_adapters/sqlserver/database_statements.rb
@@ -145,7 +145,7 @@ def build_insert_sql(insert) # :nodoc:
             returning_sql = if returning.is_a?(String)
                               returning
                             else
-                              returning.map { |column| "INSERTED.#{quote_column_name(column)}" }.join(", ")
+                              Array(returning).map { |column| "INSERTED.#{quote_column_name(column)}" }.join(", ")
                             end
             sql << " OUTPUT #{returning_sql}"
           end
diff --git a/test/cases/coerced_tests.rb b/test/cases/coerced_tests.rb
index 26ff4d15a..a5a171203 100644
--- a/test/cases/coerced_tests.rb
+++ b/test/cases/coerced_tests.rb
@@ -2487,6 +2487,17 @@ def test_insert_all_returns_requested_sql_fields_coerced
     result = Book.insert_all! [{ name: "Rework", author_id: 1 }], returning: Arel.sql("UPPER(INSERTED.name) as name")
     assert_equal %w[ REWORK ], result.pluck("name")
   end
+
+  # Need to remove index as SQL Server considers NULLs on a unique-index to be equal unlike PostgreSQL/MySQL/SQLite.
+  coerce_tests! :test_insert_with_type_casting_and_serialize_is_consistent
+  def test_insert_with_type_casting_and_serialize_is_consistent_coerced
+    connection.remove_index(:books, column: [:author_id, :name])
+
+    original_test_insert_with_type_casting_and_serialize_is_consistent
+  ensure
+    Book.where(author_id: nil, name: '["Array"]').delete_all
+    Book.lease_connection.add_index(:books, [:author_id, :name], unique: true)
+  end
 end
 
 module ActiveRecord