diff --git a/src/main/java/world/bentobox/upgrades/UpgradesDataManager.java b/src/main/java/world/bentobox/upgrades/UpgradesDataManager.java
index 0b8b514..11b25b3 100644
--- a/src/main/java/world/bentobox/upgrades/UpgradesDataManager.java
+++ b/src/main/java/world/bentobox/upgrades/UpgradesDataManager.java
@@ -300,11 +300,12 @@ private void validate() {
 	 * Unload the tier if not
 	 */
 	private void validateUpgradeTier() {
-		this.upgradeTierCache.values().forEach(tier -> {
-			if (!this.upgradeDataCache.containsKey(tier.getUpgrade())) {
-				this.addon.logWarning("Upgrade tier " + tier.getUniqueId() + " has a reference to an unknow upgrade data. It will be skiped");
-				this.upgradeTierCache.remove(tier.getUniqueId());
+		this.upgradeTierCache.entrySet().removeIf(entry -> {
+			if (!this.upgradeDataCache.containsKey(entry.getValue().getUpgrade())) {
+				this.addon.logWarning("Upgrade tier " + entry.getValue().getUniqueId() + " has a reference to an unknown upgrade data. It will be skipped");
+				return true;
 			}
+			return false;
 		});
 	}
 	
diff --git a/src/test/java/world/bentobox/upgrades/UpgradesDataManagerTest.java b/src/test/java/world/bentobox/upgrades/UpgradesDataManagerTest.java
new file mode 100644
index 0000000..243d6c5
--- /dev/null
+++ b/src/test/java/world/bentobox/upgrades/UpgradesDataManagerTest.java
@@ -0,0 +1,157 @@
+package world.bentobox.upgrades;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.when;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.util.Map;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockbukkit.mockbukkit.MockBukkit;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import world.bentobox.bentobox.BentoBox;
+import world.bentobox.bentobox.Settings;
+import world.bentobox.bentobox.database.DatabaseSetup.DatabaseType;
+import world.bentobox.upgrades.dataobjects.UpgradeData;
+import world.bentobox.upgrades.dataobjects.UpgradeTier;
+
+/**
+ * Tests for {@link UpgradesDataManager}, focusing on the validation logic
+ * that removes tiers referencing non-existent upgrade data.
+ */
+class UpgradesDataManagerTest {
+
+    @Mock
+    private UpgradesAddon addon;
+    @Mock
+    private BentoBox plugin;
+    @Mock
+    private Settings pluginSettings;
+
+    private UpgradesDataManager dataManager;
+
+    @BeforeEach
+    void setUp() {
+        MockBukkit.mock();
+        MockitoAnnotations.openMocks(this);
+
+        // Set up the BentoBox singleton that Database construction requires
+        WhiteBox.setInternalState(BentoBox.class, "instance", plugin);
+        DatabaseType value = DatabaseType.JSON;
+        when(plugin.getSettings()).thenReturn(pluginSettings);
+        when(pluginSettings.getDatabaseType()).thenReturn(value);
+        dataManager = new UpgradesDataManager(addon);
+    }
+
+    @AfterEach
+    void tearDown() {
+        MockBukkit.unmock();
+    }
+
+    /**
+     * Regression test for the ConcurrentModificationException originally thrown
+     * inside validateUpgradeTier() when removing an orphaned tier while iterating
+     * the HashMap values. The fix uses entrySet().removeIf() which is safe.
+     *
+     * Given a tier that references an upgrade data key that does not exist in the
+     * cache, calling validate should:
+     *  1. Not throw any exception (ConcurrentModificationException in particular).
+     *  2. Remove the orphaned tier from the cache.
+     */
+    @Test
+    void validateUpgradeTier_orphanedTier_removedWithoutException() throws Exception {
+        // Arrange: inject an orphaned tier (references "missing_upgrade") and no UpgradeData
+        UpgradeTier orphan = new UpgradeTier();
+        orphan.setUniqueId("test_orphan_t1");
+        orphan.setUpgrade("missing_upgrade");
+        orphan.setStartLevel(0);
+        orphan.setEndLevel(5);
+
+        Map<String, UpgradeTier> tierCache = getTierCache();
+        tierCache.put(orphan.getUniqueId(), orphan);
+
+        // Sanity-check the tier is in the cache before validation
+        assertTrue(tierCache.containsKey("test_orphan_t1"));
+
+        // Act + Assert: calling validate via reload() must not throw
+        assertDoesNotThrow(dataManager::reload);
+
+        // Assert: orphaned tier must have been removed
+        assertFalse(getTierCache().containsKey("test_orphan_t1"),
+                "Orphaned tier should be removed from the cache after validation");
+    }
+
+    /**
+     * A tier whose parent upgrade data IS in the cache must be kept.
+     */
+    @Test
+    void validateUpgradeTier_validTier_notRemoved() throws Exception {
+        // Arrange: put a matching UpgradeData in the cache
+        UpgradeData data = new UpgradeData();
+        data.setUniqueId("valid_upgrade");
+        data.setWorld("BSkyBlock");
+        getDataCache().put(data.getUniqueId(), data);
+
+        // And a tier that correctly references it
+        UpgradeTier tier = new UpgradeTier();
+        tier.setUniqueId("valid_upgrade_t1");
+        tier.setUpgrade("valid_upgrade");
+        tier.setStartLevel(0);
+        tier.setEndLevel(5);
+        getTierCache().put(tier.getUniqueId(), tier);
+
+        // Act
+        assertDoesNotThrow(dataManager::reload);
+
+        // Assert: valid tier must remain
+        assertTrue(getTierCache().containsKey("valid_upgrade_t1"),
+                "Valid tier should remain in the cache after validation");
+    }
+
+    /**
+     * Multiple orphaned tiers (the exact scenario from the reported issue) must
+     * all be removed in a single validate pass without throwing.
+     */
+    @Test
+    void validateUpgradeTier_multipleOrphanedTiers_allRemovedWithoutException() throws Exception {
+        // Arrange: two orphaned tiers with no corresponding upgrade data
+        for (int i = 1; i <= 2; i++) {
+            UpgradeTier orphan = new UpgradeTier();
+            orphan.setUniqueId("Boxed_example_orphan" + i + "_t1");
+            orphan.setUpgrade("Boxed_example_orphan" + i);
+            orphan.setStartLevel(0);
+            orphan.setEndLevel(5);
+            getTierCache().put(orphan.getUniqueId(), orphan);
+        }
+
+        // Act + Assert
+        assertDoesNotThrow(dataManager::reload);
+
+        // Assert: both orphaned tiers removed
+        assertFalse(getTierCache().containsKey("Boxed_example_orphan1_t1"));
+        assertFalse(getTierCache().containsKey("Boxed_example_orphan2_t1"));
+    }
+
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    @SuppressWarnings("unchecked")
+    private Map<String, UpgradeTier> getTierCache() throws Exception {
+        Field f = UpgradesDataManager.class.getDeclaredField("upgradeTierCache");
+        f.setAccessible(true);
+        return (Map<String, UpgradeTier>) f.get(dataManager);
+    }
+
+    @SuppressWarnings("unchecked")
+    private Map<String, UpgradeData> getDataCache() throws Exception {
+        Field f = UpgradesDataManager.class.getDeclaredField("upgradeDataCache");
+        f.setAccessible(true);
+        return (Map<String, UpgradeData>) f.get(dataManager);
+    }
+}