diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index ff766d7b..ef1425ea 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -189,7 +189,7 @@ class NetworkInterface(TaggedEC2Resource):
         self.ec2_backend = ec2_backend
         self.id = random_eni_id()
         self.device_index = device_index
-        self.private_ip_address = private_ip_address
+        self.private_ip_address = private_ip_address or random_private_ip()
         self.subnet = subnet
         self.instance = None
         self.attachment_id = None
diff --git a/tests/test_ec2/test_elastic_network_interfaces.py b/tests/test_ec2/test_elastic_network_interfaces.py
index 828f9d91..581106b4 100644
--- a/tests/test_ec2/test_elastic_network_interfaces.py
+++ b/tests/test_ec2/test_elastic_network_interfaces.py
@@ -36,7 +36,8 @@ def test_elastic_network_interfaces():
     all_enis.should.have.length_of(1)
     eni = all_enis[0]
     eni.groups.should.have.length_of(0)
-    eni.private_ip_addresses.should.have.length_of(0)
+    eni.private_ip_addresses.should.have.length_of(1)
+    eni.private_ip_addresses[0].private_ip_address.startswith('10.').should.be.true
 
     with assert_raises(EC2ResponseError) as ex:
         conn.delete_network_interface(eni.id, dry_run=True)