NoSuchFieldException when using getDeclaredField() and field exists

java.lang.NoSuchFieldException: c

at java.lang.Class.getDeclaredField(Unknown Source)

at ru.onlymc.OnlyMZ.CustomEntityType.getPrivateStatic(CustomEntityType.java:177)

Method:

private static Object getPrivateStatic(Class clazz, String f) throws Exception {
    Field field = clazz.getDeclaredField(f);
    field.setAccessible(true);
    return field.get(null);
}

Calling:

private static void a(Class paramClass, String paramString, int paramInt) {
    try {
        ((Map) getPrivateStatic(sg.class, "c")).put(paramString, paramClass);
        //...
    } catch (Exception exc) {
        exc.printStackTrace();
    }
}

sg.class (from decompile to sure about required fields really exists):

private static Map c = new HashMap();
private static Map d = new HashMap();
private static Map e = new HashMap();
private static Map f = new HashMap();
private static Map g = new HashMap();
public static HashMap a = new LinkedHashMap();