SSMStore - Must be <type 'str'>. Actual type <type 'unicode'>.

**Stacker: `stacker 1.7.0`
Boto: `boto3 1.9.187`
Botocore: `botocore 1.12.187`**

I'm using the ECS base for generating Fargate tasks. (https://github.com/cloudtools/stacker_blueprints/blob/master/stacker_blueprints/ecs.py)

I use ${ssmstore _____} throughout my stacks, but always in the Environment: blocks. Most frequently I store the path of the SSM parameter in an environment file, and call it like ${ssmstore ${example1_path}}.

I'm trying to use SSMStore in the Image: block of ECS, but get an error.
``Image: ${ecr_url}/company/example.api:${ssmstore eu-west-2@/prod/api/example-api/latesttag}`` - (Where latesttag in SSM = "ref-bh43r78gf9fge9hd")

> [2019-07-10T09:38:36] Value for variable Image must be of type . Actual type: .
Traceback (most recent call last):
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/plan.py", line 93, in _run_once
status = self.fn(self.stack, status=self.status)
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/actions/build.py", line 326, in _launch_stack
stack.resolve(self.context, self.provider)
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/stack.py", line 194, in resolve
self.blueprint.resolve_variables(self.variables)
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/blueprints/base.py", line 452, in resolve_variables
self.name
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/blueprints/base.py", line 226, in resolve_variable
value = validate_variable_type(var_name, var_type, value)
File "/usr/local/lib/python2.7/site-packages/stacker-1.7.0-py2.7.egg/stacker/blueprints/base.py", line 147, in validate_variable_type
"type: %s." % (var_name, var_type, type(value))
ValueError: Value for variable Image must be of type . Actual type: .
[2019-07-10T09:38:36] fargate-example-api: failed (Value for variable Image must be of type . Actual type: .)

I've tried manually typing the variable in AWS console, and through awscli. It seems to just be ssmstore inside the Images block, as there is no issues in the Environment blocks.