ResolveGrammarActivity doesn't like ma-data SyncConfig-id

I tried to set up a workflow that grabs the MA ID and display name whenever an MA changes, but the grammar resolver doesn't like the SyncConfig-id attribute of the ma-data object.

This seemed so close to an error I troubleshot before, but this time I have no control over the attribute type (which is an indexed string, BTW).

Grammar expression: [//Target/SyncConfig-id]

Error:
System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.ResourceManagement.WFActivities.Resolver.GetDisplayStringFromGuid(Guid id, String[] expansionAttributes) at Microsoft.ResourceManagement.WFActivities.Resolver.ReplaceGuidWithTemplatedString(Match m) at System.Text.RegularExpressions.RegexReplacement.Replace(MatchEvaluator evaluator, Regex regex, String input, Int32 count, Int32 startat) at System.Text.RegularExpressions.Regex.Replace(String input, MatchEvaluator evaluator) at Microsoft.ResourceManagement.WFActivities.Resolver.GetStringAttributeValue(Object attribute) at Microsoft.ResourceManagement.WFActivities.Resolver.ResolveEvaluatorWithoutAntiXSS(String match, ResolverOptions resolveOptions) at Microsoft.ResourceManagement.WFActivities.Resolver.ResolveEvaluatorForWithAntiXSS(String match, ResolverOptions resolveOptions) at Microsoft.ResourceManagement.WFActivities.Resolver.ReplaceMatches(String input, Boolean useAntiXssEncoding, ResolverOptions resolveOptions) at Microsoft.ResourceManagement.Workflow.Hosting.ResolverEvaluationServiceImpl.ResolveLookupGrammar(Guid requestId, Guid targetId, Guid actorId, Dictionary`2 workflowDictionary, Boolean encodeForHTML, String expression) at Microsoft.ResourceManagement.Workflow.Activities.ResolveGrammarActivity.Execute(ActivityExecutionContext executionContext) at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(T activity, ActivityExecutionContext executionContext) at System.Workflow.ComponentModel.ActivityExecutor`1.Execute(Activity activity, ActivityExecutionContext executionContext) at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime) at System.Workflow.Runtime.Scheduler.Run()