Test Runner Not Detecting Equal Lists of Strings

When running the test CqlNullologicalOperatorsTest:Coalesce:CoalesceFirstList, the test runner reports:

Test CqlNullologicalOperatorsTest:Coalesce:CoalesceFirstList status: fail expected: {'a'} actual: a

My engine is returning the following result:

{
  "resourceType": "Parameters",
  "parameters": [
    {
      "extension": [
        {
          "url": "http://hl7.org/fhir/StructureDefinition/cqf-cqlType",
          "valueString": "List<System.String>"
        }
      ],
      "name": "return",
      "valueString": "a"
    }
  ]
}

According to the CQL IG FHIR type mapping, System.String maps to FHIR.string. According to the $cql operation, the parameter should be repeated for each element in the list (but in this case there is only one). So... I believe my engine is returning the correct result to represent { 'a' }.