java.lang.IllegalArgumentException: Can not set java.lang.String field com.w.state.ProviderSearch.config_URL to java.lang.String

I fail to fill an object with reflection. can you please tell why?

public void setConfig(WyPreferences wyPreferences, String category, String key, String val) {
    String normalizedCategory = stringUtils.normalizeFieldNameToCamelCase(category);
    String normalizedKey = stringUtils.normalizeFieldNameToUnderScore(key);

    try {
        Field field1 = WyPreferences.class.getDeclaredField(normalizedCategory);
        Object object1 = field1.get(wyPreferences);
        if (object1 == null)
        {
            object1 = field1.getType().newInstance();
        }
        Field field2 = field1.getType().getDeclaredField(normalizedKey);
        Object object2 = field2.get(object1);
        if (object2 == null)
        {
            object2 = field2.getType().newInstance();
        }
        field2.set(object2, val);
        field1.set(object1, object2);
    } catch (Exception ex) {
        String a = "";
    }

category = providerSearch

key =config_URL

where val = "bla"

and with this class:

public class WyPreferences {

    public ProviderSearch providerSearch;

    public WyPreferences() {
        this.providerSearch = new ProviderSearch();
    }
}



    public ProviderSearch() {
        this.config_URL = "";
        this.gas_popup_feature_enabled = "";
    }

my code fails on field2.set(object2, val);

with this exception

java.lang.IllegalArgumentException: Can not set java.lang.String field com.w.state.ProviderSearch.config_URL to java.lang.String