reproduction steps.
Add two tags to advanced ami settings in jenkins
tag one key "Name" value "CI-slave"
tag two both key and value empty.

Set first instance cap to null
Apply settings.
Build task.

The following errors appear in the logs and no nodes are created in jenkins however ec2 does create the instances which are then not tracked and eventually terminated/stopped.

Oct 2, 2013 10:47:31 PM hudson.slaves.NodeProvisioner update
WARNING: Provisioned slave Ci Master AMI (ami-e31f82d9) failed to launch
Status Code: 400, AWS Service: AmazonEC2, AWS Request ID: 50ae365d-41e2-4d5a-a068-bf7cd6848ec6, AWS Error Code: InvalidParameterValue, AWS Error Message: Tag key must have a value
at com.amazonaws.http.AmazonHttpClient.handleErrorResponse(AmazonHttpClient.java:614)
at com.amazonaws.http.AmazonHttpClient.executeHelper(AmazonHttpClient.java:312)
at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:165)
at com.amazonaws.services.ec2.AmazonEC2Client.invoke(AmazonEC2Client.java:6047)
at com.amazonaws.services.ec2.AmazonEC2Client.createTags(AmazonEC2Client.java:3343)
at hudson.plugins.ec2.SlaveTemplate.provision(SlaveTemplate.java:322)
at hudson.plugins.ec2.EC2Cloud$1.call(EC2Cloud.java:333)
at hudson.plugins.ec2.EC2Cloud$1.call(EC2Cloud.java:329)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:662)
Oct 2, 2013 10:47:32 PM hudson.plugins.ec2.EC2Cloud addProvisionedSlave
INFO: Provisioning for AMI ami-e31f82d9; Estimated number of total slaves: 83; Estimated number of slaves for ami ami-e31f82d9: 3
Launching ami-e31f82d9 for template Ci Master AMI
Oct 2, 2013 10:47:32 PM hudson.slaves.NodeProvisioner update
INFO: Started provisioning Ci Master AMI (ami-e31f82d9) from ec2-ap-southeast-2 with 2 executors. Remaining excess workload:-1.0
Looking for existing instances: {InstanceIds: [], Filters: [

{Name: image-id, Values: [ami-e31f82d9], }

,

{Name: availability-zone, Values: [ap-southeast-2a], }

,

{Name: subnet-id, Values: [subnet-90da38f9], }

,

{Name: instance.group-id, Values: [sg-708c9f1c], }

,

{Name: key-name, Values: [au-dev], }

,

{Name: instance-type, Values: [m1.medium], }

,

{Name: tag:Name, Values: [ci-slave], }

,

{Name: tag:, Values: [], }

,

{Name: instance-state-name, Values: [stopped, stopping], }

], }