Cannot set readonly property - when modifing attributes

[Bill Lee]

I am attempting to modify an attribute based on a criteria. When I attempt the following, I get

Exception in thread "main" groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: value for class: java.lang.String

allRecords.findAll{
it.'@ZipCode'.equals('60245')
}each{
it.'@State'.value = 'IL'
}

Does anyone know how to surpass this exception? Thank.s