diff --git a/pg/format.js b/pg/format.js
index 7905bea..50112ac 100644
--- a/pg/format.js
+++ b/pg/format.js
@@ -6,6 +6,8 @@ const escape = require('./escape');
  * @returns { string }
  */
 module.exports = function(statement, obj){
+    obj = JSON.parse(JSON.stringify(obj)); // clone obj to avoid mutations
+
     const array_mode = Array.isArray(obj);
     if(array_mode && obj.length === 0)
         throw new Error('array should have at least one element');
@@ -46,4 +48,4 @@ module.exports = function(statement, obj){
             values += (values.length > 0 ? ',' : '') + key + '=' + obj[key];
         return statement.replace('?', values);
     }
-}
\ No newline at end of file
+}
diff --git a/pg/format.test.js b/pg/format.test.js
index a86e2d8..f5ea80b 100644
--- a/pg/format.test.js
+++ b/pg/format.test.js
@@ -8,4 +8,9 @@ tap.equal(format('INSERT INTO customer ?', { fullname: 'Test', balance: 1 }), `I
 tap.equal(format('UPDATE customer SET ?', { last_seen: 'NOW()', visits: 3 }), `UPDATE customer SET last_seen=NOW(),visits=3`);
 tap.equal(format('UPDATE customer SET ?', { fullname: 'Test', '!visits': '(SELECT COUNT(*) FROM customer_visits)' }), `UPDATE customer SET fullname='Test',visits=(SELECT COUNT(*) FROM customer_visits)`);
 
-tap.equal(format('INSERT INTO customer ?', [{ fullname: 'Test', balance: 1 }, { fullname: 'Test 2', balance: 3 }]), `INSERT INTO customer (fullname,balance) VALUES ('Test',1),('Test 2',3)`);
\ No newline at end of file
+tap.equal(format('INSERT INTO customer ?', [{ fullname: 'Test', balance: 1 }, { fullname: 'Test 2', balance: 3 }]), `INSERT INTO customer (fullname,balance) VALUES ('Test',1),('Test 2',3)`);
+
+const input = [{a: 1, b: null}];
+const backup = JSON.parse(JSON.stringify(input));
+format('INSERT INTO customer ?', input);
+tap.same(input, backup, 'format should not mutate its params');