#2877 - Ensure NetworkInterfaces are assigned to the default Subnet

moto/ec2/models.py

@@ -775,7 +775,14 @@ class Instance(TaggedEC2Resource, BotoInstance):
                 if "SubnetId" in nic:
                     subnet = self.ec2_backend.get_subnet(nic["SubnetId"])
                 else:
-                    subnet = None
+                    # Get default Subnet
+                    subnet = [
+                        subnet
+                        for subnet in self.ec2_backend.get_all_subnets(
+                            filters={"availabilityZone": self._placement.zone}
+                        )
+                        if subnet.default_for_az
+                    ][0]
 
                 group_id = nic.get("SecurityGroupId")
                 group_ids = [group_id] if group_id else []