Issue using @MapConstructor and @NamedVariant

I have a static method marked with @NamedVariant, that takes a "Configuration" object as a parameter, which is marked with @NamedDelegate. The "Configuration" object is defined as a static inner class marked with @Immutable.
 
My Class:

class CsvRowMapper {
    private final Settings settings
    @NamedVariant
    CsvRowMapper(Settings settings) {
        this.settings = settings
    }
    @NamedVariant
    static CsvRowMapper parse(@NamedDelegate Settings settings, Reader reader) {
        new CsvRowMapper(settings).parse(reader)
    }
    CsvRowMapper parse(Reader source) {
        // do work here
        return this
    }
    @Immutable
    static class Settings {
        String separator = ','
        boolean headers = true
        int headersRow = 0
        int firstDataRow = 1
    }
}

 
I've highlighted the important parts. The issue is when I use this class like:

CsvRowMapper.parse(*separator: '\t'*, tsvFileReader)

 
I get an GroovyCastException because it's trying to set headersRow or firstDataRow to null:
 

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '\{separator= , headers=null, headersRow=null, firstDataRow=null}'
  with class 'java.util.LinkedHashMap' to class 'CsvRowMapper$Settings'
  due to: org.codehaus.groovy.runtime.typehandling.GroovyCastException:
    Cannot cast object 'null' with class 'null' to class 'int'. Try 'java.lang.Integer' instead

 
Even though there are default values for each setting.
 
I loaded up this into GroovyConsole, and I think the issue is that the @NameVariant version of the `parse` method is creating a map with ALL keys for the @NamedDelegate Settings class (See NamedVariant.png).
 
Furthermore the @Immutable annotation adds @MapConstructor to my Settings class, which only checks if the given map contains the `key`, not also if it has a `value` (See MapConstructor.png).
 
The @NamedVariant passes a map containing null values for all keys not supplied during method execution, and @MapConstructor only checks if the given map has the key, not a value.