how to fix operator + undefined for the arguments type(s) int, object

Question

i have source code call as PhpArray Class from JavaBridge

public final class PhpArray extends AbstractMap
{
    private TreeMap t;
    private HashMap m;
    
    public PhpArray() {
        this.t = new TreeMap(Request.PHP_ARRAY_KEY_COMPARATOR);
        this.m = null;
    }
    
    @Override
    public Object put(final Object key, final Object value) {
        if (this.m != null) {
            return this.m.put(key, value);
        }
        try {
            return this.t.put(key, value);
        }
        catch (ClassCastException e) {
            this.m = new HashMap(this.t);
            this.t = null;
            return this.m.put(key, value);
        }
    }
    
    @Override
    public Set entrySet() {
        if (this.t != null) {
            return this.t.entrySet();
        }
        return this.m.entrySet();
    }
    
    public int arraySize() {
        if (this.t == null) {
            throw new IllegalArgumentException("The passed PHP \"array\" is not a sequence but a dictionary");
        }
        if (this.t.size() == 0) {
            return 0;
        }
        return 1 + this.t.lastKey();
    }
}

i got an error in this code

public int arraySize() {
            if (this.t == null) {
                throw new IllegalArgumentException("The passed PHP \"array\" is not a sequence but a dictionary");
            }
            if (this.t.size() == 0) {
                return 0;
            }
            return 1 + this.t.lastKey();
        }

error which i got are

operator + undefined for the arguments type(s) int, object

from my lastKey() we call this methods

public K lastKey() {
        return key(getLastEntry());
    }

and here for getLastEntry

final Entry<K,V> getLastEntry() {
        Entry<K,V> p = root;
        if (p != null)
            while (p.right != null)
                p = p.right;
        return p;
    }

i want to know how to fix it ? i have try add after TreeMap like this TreeMap<String,String> i got still error. so i want to know how to fix it

from this call i see this is for return int

protected static final IntegerComparator PHP_ARRAY_KEY_COMPARATOR;

after i change my code as like this return 1 + (Integer)this.t.lastKey(); this code not get error again... i want to know suggestion from this forum is it ok for this code ?