Trigger lambda on ECR PutImage event

Question

I'm having trouble understanding why a Cloudwatch event rule is not firing. I've followed this related question and did the following.

1. Created a Cloudtrail which sends events to a Cloudwatch log

2. Created the following CloudWatch event rule:

   {
     "detail-type": [
       "AWS write API Call via CloudTrail"
     ],
     "source": [
       "aws.ecr"
     ],
     "detail": {
       "eventSource": [
         "ecr.amazonaws.com"
       ],
       "eventName": [
         "PutImage"
       ]
     }
   }
   

3. Created a lambda to be invoked by this rule.

I can verify that in my Cloudwatch log group (set up to accept events from Cloudtrail) I am seeing the PutImage event. However, the lambda never fires and the rule metrics show that it is never triggered. I am assuming at this point the rule must be faulty (I would expect to see the rule triggered even if the lambda is faulty) but I can't see what additional logic is required. Is it necessary to link my event to a particular log group?