diff --git a/src/main/java/world/bentobox/bentobox/api/panels/builders/PanelBuilder.java b/src/main/java/world/bentobox/bentobox/api/panels/builders/PanelBuilder.java
index 424129b88..db919213d 100644
--- a/src/main/java/world/bentobox/bentobox/api/panels/builders/PanelBuilder.java
+++ b/src/main/java/world/bentobox/bentobox/api/panels/builders/PanelBuilder.java
@@ -70,14 +70,15 @@ public class PanelBuilder {
     }
 
     /**
-     * Get the next free slot number
+     * Get the next free slot number after the largest slot
      * @return next slot number, or -1 in case none has been found.
      */
     public int nextSlot() {
-        for (int i = 0 ; i < (size == 0 ? 54 : size) ; i++) {
-            if (!slotOccupied(i)) return i;
-        }
-        return -1;
+        return items.isEmpty() ? 0 : items.lastKey() + 1;
+        //for (int i = 0 ; i < (size == 0 ? 54 : size) ; i++) {
+        //    if (!slotOccupied(i)) return i;
+        //}
+        //return -1;
     }
 
     /**
diff --git a/src/test/java/world/bentobox/bentobox/api/panels/builders/PanelBuilderTest.java b/src/test/java/world/bentobox/bentobox/api/panels/builders/PanelBuilderTest.java
new file mode 100644
index 000000000..e7373ee19
--- /dev/null
+++ b/src/test/java/world/bentobox/bentobox/api/panels/builders/PanelBuilderTest.java
@@ -0,0 +1,164 @@
+/**
+ * 
+ */
+package world.bentobox.bentobox.api.panels.builders;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import org.bukkit.Bukkit;
+import org.bukkit.inventory.Inventory;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mockito;
+import org.powermock.api.mockito.PowerMockito;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+import org.powermock.modules.junit4.PowerMockRunner;
+
+import world.bentobox.bentobox.api.panels.PanelItem;
+import world.bentobox.bentobox.api.panels.PanelListener;
+import world.bentobox.bentobox.api.user.User;
+
+/**
+ * @author tastybento
+ *
+ */
+@RunWith(PowerMockRunner.class)
+@PrepareForTest({Bukkit.class })
+public class PanelBuilderTest {
+
+    /**
+     * @throws java.lang.Exception
+     */
+    @Before
+    public void setUp() throws Exception {
+        PowerMockito.mockStatic(Bukkit.class);
+        Inventory inv = mock(Inventory.class);
+        when(Bukkit.createInventory(Mockito.any(), Mockito.any(), Mockito.any())).thenReturn(inv);
+        
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#name(java.lang.String)}.
+     */
+    @Test
+    public void testName() {
+        PanelBuilder pb = new PanelBuilder();
+        assertTrue(pb.name("test") instanceof PanelBuilder);    
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#item(world.bentobox.bentobox.api.panels.PanelItem)}.
+     */
+    @Test
+    public void testItemPanelItem() {
+        PanelItem pi = mock(PanelItem.class);
+        PanelBuilder pb = new PanelBuilder();
+        pb = pb.item(pi);
+        // Add multiple items
+        pb = pb.item(pi);
+        pb = pb.item(pi);
+        pb = pb.item(pi);
+        
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#item(int, world.bentobox.bentobox.api.panels.PanelItem)}.
+     */
+    @Test
+    public void testItemIntPanelItem() {
+        PanelItem pi = mock(PanelItem.class);
+        PanelBuilder pb = new PanelBuilder();
+        pb = pb.item(0, pi);
+        // Add multiple items
+        pb = pb.item(1, pi);
+        pb = pb.item(10, pi);
+        pb = pb.item(20, pi);
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#size(int)}.
+     */
+    @Test
+    public void testSize() {
+        PanelBuilder pb = new PanelBuilder();
+        pb = pb.size(45);
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#user(world.bentobox.bentobox.api.user.User)}.
+     */
+    @Test
+    public void testUser() {
+        User user = mock(User.class);
+        PanelBuilder pb = new PanelBuilder();
+        pb = pb.user(user);
+        // Change user
+        User user2 = mock(User.class);
+        pb = pb.user(user2);
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#listener(world.bentobox.bentobox.api.panels.PanelListener)}.
+     */
+    @Test
+    public void testListener() {
+        PanelBuilder pb = new PanelBuilder();
+        PanelListener listener = mock(PanelListener.class);
+        pb = pb.listener(listener);
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#nextSlot()}.
+     */
+    @Test
+    public void testNextSlot() {
+        PanelItem pi = mock(PanelItem.class);
+        PanelBuilder pb = new PanelBuilder();
+        assertEquals(0, pb.nextSlot());
+        pb = pb.item(0, pi);
+        assertEquals(1, pb.nextSlot());
+        // Add multiple items
+        pb = pb.item(1, pi);
+        assertEquals(2, pb.nextSlot());
+        pb = pb.item(10, pi);
+        assertEquals(11, pb.nextSlot());
+        pb = pb.item(20, pi);
+        assertEquals(21, pb.nextSlot());
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#slotOccupied(int)}.
+     */
+    @Test
+    public void testSlotOccupied() {
+        PanelItem pi = mock(PanelItem.class);
+        PanelBuilder pb = new PanelBuilder();
+        assertEquals(0, pb.nextSlot());
+        assertFalse(pb.slotOccupied(0));
+        pb = pb.item(0, pi);
+        assertTrue(pb.slotOccupied(0));
+        
+        // Add multiple items
+        pb = pb.item(1, pi);
+        assertTrue(pb.slotOccupied(1));
+        pb = pb.item(10, pi);
+        assertTrue(pb.slotOccupied(10));
+        pb = pb.item(20, pi);
+        assertTrue(pb.slotOccupied(20));
+    }
+
+    /**
+     * Test method for {@link world.bentobox.bentobox.api.panels.builders.PanelBuilder#build()}.
+     */
+    @Test
+    public void testBuild() {
+        PanelBuilder pb = new PanelBuilder();
+        pb.build();
+     }
+
+}