@CS flow typing incorrectly casting to map at runtime

import groovy.json.*
import groovy.transform.*

@CompileStatic
class Bar {
    private Writable renderTemplate(Object o, Map args) {
    
    }
    private boolean isSimpleType(Class type) {
        return type == String
    }
    def foo(Map map, Map arguments) {
    
        def writable = new Writable() {
            @Override
            Writer writeTo(Writer out) throws IOException {
                for(entry in map.entrySet()) {
                   def value = entry.value
                   if(isSimpleType(value.getClass()) || (value instanceof Map)) {
                        out.append(JsonOutput.toJson(value))
                    }
                }
                return out
            }
        }
    }
}

writable = new Bar().foo([one:'two'],[foo:'bar'])

sw = new StringWriter()
writable.writeTo(sw)
println sw

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'two' with class 'java.lang.String' to class 'java.util.Map'
	at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.continueCastOnSAM(DefaultTypeTransformation.java:405)
	at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.continueCastOnNumber(DefaultTypeTransformation.java:319)
	at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.castToType(DefaultTypeTransformation.java:232)