"Unknown value type class java.lang.String" with [foo()] rail syntax

class R(rank:nat) {
    protected def this(rank:nat) = property(rank);
}

class BD(rank:nat) {

    static type R(rank:nat) = R{self.rank==rank};

    incomplete public def get(): R(rank);

    val rs: Rail[R] = [get()];
    //workaround: val rs:Rail[R] = Rail.makeVal[R](1, (nat)=>get());

    protected def this(rank:nat) = property(rank);
}

Exception in thread "main" java.lang.RuntimeException: Unknown value type class java.lang.String
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.serializeName(X10PrettyPrinterVisitor.java:1424)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.serializeTerm(X10PrettyPrinterVisitor.java:1366)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.serializeTerm(X10PrettyPrinterVisitor.java:1346)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.serializeTerm(X10PrettyPrinterVisitor.java:1269)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.serializeConstraint(X10PrettyPrinterVisitor.java:1225)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.access$200(X10PrettyPrinterVisitor.java:184)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor$RuntimeTypeExpander.expand(X10PrettyPrinterVisitor.java:3155)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.prettyPrint(X10PrettyPrinterVisitor.java:2982)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.dumpRegex(X10PrettyPrinterVisitor.java:3024)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.dump(X10PrettyPrinterVisitor.java:3000)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.access$500(X10PrettyPrinterVisitor.java:184)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor$Template.expand(X10PrettyPrinterVisitor.java:3216)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor$Template.expand(X10PrettyPrinterVisitor.java:3213)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:1747)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:222)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.visit.Translator.print(Translator.java:131)
at polyglot.ext.x10.visit.X10Translator.print(X10Translator.java:52)
at polyglot.ast.Node_c.print(Node_c.java:405)
at polyglot.ast.Expr_c.printSubExpr(Expr_c.java:147)
at polyglot.ast.Expr_c.printSubExpr(Expr_c.java:120)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:1501)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:259)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.prettyPrint(X10PrettyPrinterVisitor.java:2984)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.dumpRegex(X10PrettyPrinterVisitor.java:3024)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.emitNativeAnnotation(X10PrettyPrinterVisitor.java:1783)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:1796)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:264)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.visit.Translator.print(Translator.java:131)
at polyglot.ext.x10.visit.X10Translator.print(X10Translator.java:52)
at polyglot.ast.Node_c.print(Node_c.java:405)
at polyglot.ast.FieldDecl_c.prettyPrint(FieldDecl_c.java:412)
at polyglot.ast.JL_c.prettyPrint(JL_c.java:257)
at polyglot.ast.Node_c.translate(Node_c.java:411)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:211)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:1987)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visit(X10DelegatingVisitor.java:382)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:218)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.visit.Translator.print(Translator.java:131)
at polyglot.ext.x10.visit.X10Translator.print(X10Translator.java:52)
at polyglot.ast.Node_c.print(Node_c.java:405)
at polyglot.ast.Node_c.printBlock(Node_c.java:390)
at polyglot.ast.ClassBody_c.prettyPrint(ClassBody_c.java:186)
at polyglot.ast.JL_c.prettyPrint(JL_c.java:257)
at polyglot.ast.Node_c.translate(Node_c.java:411)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:211)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visit(X10DelegatingVisitor.java:306)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visit(X10DelegatingVisitor.java:314)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visit(X10DelegatingVisitor.java:315)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visit(X10DelegatingVisitor.java:316)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:284)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.visit.Translator.print(Translator.java:131)
at polyglot.ext.x10.visit.X10Translator.print(X10Translator.java:52)
at polyglot.ast.Node_c.print(Node_c.java:405)
at polyglot.ext.x10.visit.X10PrettyPrinterVisitor.visit(X10PrettyPrinterVisitor.java:1008)
at polyglot.ext.x10.visit.X10DelegatingVisitor.visitAppropriate(X10DelegatingVisitor.java:282)
at polyglot.ext.x10.ast.X10DelFactory_c$TD.translate(X10DelFactory_c.java:45)
at polyglot.visit.Translator.translateTopLevelDecl(Translator.java:247)
at polyglot.ext.x10.visit.X10Translator.translateSource(X10Translator.java:99)
at polyglot.visit.Translator.translate(Translator.java:142)
at polyglot.frontend.OutputGoal.runTask(OutputGoal.java:35)
at polyglot.frontend.Scheduler.runPass(Scheduler.java:321)
at polyglot.frontend.AbstractGoal_c.run(AbstractGoal_c.java:88)
at polyglot.types.LazyRef_c.get(LazyRef_c.java:45)
at polyglot.frontend.AbstractGoal_c.run(AbstractGoal_c.java:44)
at polyglot.types.LazyRef_c.get(LazyRef_c.java:45)
at polyglot.frontend.AbstractGoal_c.run(AbstractGoal_c.java:44)
at polyglot.types.LazyRef_c.get(LazyRef_c.java:45)
at polyglot.frontend.Scheduler.attempt(Scheduler.java:233)
at polyglot.frontend.Scheduler.runToCompletion(Scheduler.java:170)
at polyglot.frontend.Compiler.compile(Compiler.java:185)
at polyglot.frontend.Compiler.compileFiles(Compiler.java:149)
at polyglot.main.Main.start(Main.java:119)
at polyglot.main.Main.start(Main.java:74)
at polyglot.main.Main.main(Main.java:252)