diff --git a/gigamap/gigamap/src/main/java/org/eclipse/store/gigamap/types/AbstractCompositeBitmapIndex.java b/gigamap/gigamap/src/main/java/org/eclipse/store/gigamap/types/AbstractCompositeBitmapIndex.java
index ffe1a5a6..1312da20 100644
--- a/gigamap/gigamap/src/main/java/org/eclipse/store/gigamap/types/AbstractCompositeBitmapIndex.java
+++ b/gigamap/gigamap/src/main/java/org/eclipse/store/gigamap/types/AbstractCompositeBitmapIndex.java
@@ -525,13 +525,44 @@ protected void addSubResult(
 		subResultAnds.add(new BitmapResult.ChainOr(subResults.toArray(BitmapResult.class)));
 	}
 	
+
 	final void internalHandleChanged(final KS oldKeys, final long entityId, final KS newKeys)
 	{
+		// Ensure sub-indices array is large enough for newKeys
 		this.ensureSubIndices(newKeys);
 
-		for(final Sub<E, KS, K> subIndex : this.subIndices)
+		// Iterate all sub-indices and handle each position based on old/new key presence
+		for(int i = 0; i < this.subIndices.length; i++)
 		{
-			subIndex.internalHandleChanged(oldKeys, entityId, newKeys);
+			// Check if old/new keys are empty at this sub-index position
+			// This guard is critical: when oldKeys is shorter than subIndices (e.g. NULL() = Object[1]
+			// vs decomposed Instant = Object[6]), positions beyond oldKeys.length must be treated as
+			// "new additions" not "changes" to avoid ArrayIndexOutOfBoundsException.
+			final boolean oldEmpty = this.isEmpty(oldKeys, i);
+			final boolean newEmpty = this.isEmpty(newKeys, i);
+
+			if(oldEmpty && newEmpty)
+			{
+				// Both empty: nothing to do
+				continue;
+			}
+
+			final Sub<E, KS, K> subIndex = this.subIndices[i];
+			if(oldEmpty)
+			{
+				// Old key didn't exist at this position: ADD
+				subIndex.internalAddToEntry(entityId, newKeys);
+			}
+			else if(newEmpty)
+			{
+				// New key no longer relevant: REMOVE
+				subIndex.internalRemove(entityId, oldKeys);
+			}
+			else
+			{
+				// Both exist: HANDLE CHANGE
+				subIndex.internalHandleChanged(oldKeys, entityId, newKeys);
+			}
 		}
 		this.markStateChangeChildren();
 	}
diff --git a/gigamap/gigamap/src/test/java/org/eclipse/store/gigamap/indexer/CompositeIndexNullHandlingTest.java b/gigamap/gigamap/src/test/java/org/eclipse/store/gigamap/indexer/CompositeIndexNullHandlingTest.java
new file mode 100644
index 00000000..7904c058
--- /dev/null
+++ b/gigamap/gigamap/src/test/java/org/eclipse/store/gigamap/indexer/CompositeIndexNullHandlingTest.java
@@ -0,0 +1,207 @@
+package org.eclipse.store.gigamap.indexer;
+
+/*-
+ * #%L
+ * EclipseStore GigaMap
+ * %%
+ * Copyright (C) 2023 - 2026 MicroStream Software
+ * %%
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ * #L%
+ */
+
+
+import org.eclipse.store.gigamap.types.GigaMap;
+import org.eclipse.store.gigamap.types.IndexerInstant;
+import org.eclipse.store.storage.embedded.types.EmbeddedStorage;
+import org.eclipse.store.storage.embedded.types.EmbeddedStorageManager;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.nio.file.Path;
+import java.time.Instant;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Test for GitHub issue: ArrayIndexOutOfBoundsException in SubBitmapIndexHashing
+ * when updating entity from null to non-null composite key value.
+ * <p>
+ * Root cause: {@code AbstractCompositeBitmapIndex.internalHandleChanged()} called
+ * {@code ensureSubIndices(newKeys)} which grew the sub-index array (e.g. from 1 to 6),
+ * then iterated <strong>all</strong> sub-indices calling
+ * {@code subIndex.internalHandleChanged(oldKeys, ...)}.
+ * Each sub-index called {@code internalLookupEntry(oldKeys)} → {@code indexEntity(oldKeys)} →
+ * {@code index(oldKeys)} → {@code oldKeys[this.position]}.
+ * When {@code oldKeys} was the {@code NULL()} sentinel (length 1) but
+ * {@code this.position >= 1}, it threw {@code ArrayIndexOutOfBoundsException}.
+ * <p>
+ * Fix: {@code internalHandleChanged} now checks {@code isEmpty(oldKeys, i)} and
+ * {@code isEmpty(newKeys, i)} per sub-index, dispatching to {@code internalAddToEntry}
+ * or {@code internalRemove} instead of unconditionally calling {@code internalHandleChanged}.
+ */
+public class CompositeIndexNullHandlingTest
+{
+
+	@TempDir
+	Path tempDir;
+
+	/**
+	 * Entity with an optional {@link Instant} field — mirrors real-world scenario
+	 * like Booking with nullable {@code expiresAt}, {@code billingReportedAt}, etc.
+	 */
+	static final class Event
+	{
+		final long id;
+		Instant timestamp; // initially null, set on update
+
+		Event(final long id)
+		{
+			this.id = id;
+			this.timestamp = null;
+		}
+	}
+
+	static final class EventIndexer extends IndexerInstant.Abstract<Event>
+	{
+		@Override
+		protected Instant getInstant(final Event entity)
+		{
+			return entity.timestamp;
+		}
+	}
+
+	@Test
+	void nullToNonNullCompositeKeyUpdateShouldWork()
+	{
+		final GigaMap<Event> map = GigaMap.New();
+		final EventIndexer indexer = new EventIndexer();
+		map.index().bitmap().ensure(indexer);
+
+		// Add entity with null timestamp → composite key = NULL() = Object[1]
+		final Event event = new Event(1L);
+		map.add(event);
+		assertEquals(1, map.size());
+
+		// Update to set non-null timestamp → composite key = Object[6]
+		// Before fix: ArrayIndexOutOfBoundsException (Index 1 out of bounds for length 1)
+		assertDoesNotThrow(() ->
+			map.update(event, e -> e.timestamp = Instant.parse("2025-06-15T10:30:00Z"))
+		);
+
+		assertEquals(1, map.size());
+	}
+
+	@Test
+	void nonNullToNullCompositeKeyUpdateShouldWork()
+	{
+		final GigaMap<Event> map = GigaMap.New();
+		final EventIndexer indexer = new EventIndexer();
+		map.index().bitmap().ensure(indexer);
+
+		// Add entity with non-null timestamp → composite key = Object[6]
+		final Event event = new Event(1L);
+		event.timestamp = Instant.parse("2025-06-15T10:30:00Z");
+		map.add(event);
+
+		// Update to null timestamp → composite key = NULL() = Object[1]
+		assertDoesNotThrow(() ->
+			map.update(event, e -> e.timestamp = null)
+		);
+
+		assertEquals(1, map.size());
+	}
+
+	@Test
+	void nullToNonNullCompositeKeyUpdateWithPersistenceShouldWork()
+	{
+		final GigaMap<Event> map = GigaMap.New();
+		final EventIndexer indexer = new EventIndexer();
+		map.index().bitmap().ensure(indexer);
+
+		// Add entity with null timestamp
+		final Event event = new Event(1L);
+		map.add(event);
+
+		try (EmbeddedStorageManager manager = EmbeddedStorage.start(map, this.tempDir))
+		{
+			// Update to set non-null timestamp
+			assertDoesNotThrow(() ->
+				map.update(event, e -> e.timestamp = Instant.parse("2025-06-15T10:30:00Z"))
+			);
+
+			assertEquals(1, map.size());
+			map.store();
+		}
+
+		// Verify persistence
+		try (EmbeddedStorageManager manager = EmbeddedStorage.start(this.tempDir))
+		{
+			final GigaMap<Event> loadedMap = (GigaMap<Event>) manager.root();
+			assertEquals(1, loadedMap.size());
+
+			final Event loadedEvent = loadedMap.query().findFirst().get();
+			assertNotNull(loadedEvent.timestamp);
+			assertEquals(Instant.parse("2025-06-15T10:30:00Z"), loadedEvent.timestamp);
+		}
+	}
+
+	@Test
+	void multipleNullToNonNullUpdatesShouldWork()
+	{
+		final GigaMap<Event> map = GigaMap.New();
+		final EventIndexer indexer = new EventIndexer();
+		map.index().bitmap().ensure(indexer);
+
+		// Add multiple entities with null timestamps
+		final Event event1 = new Event(1L);
+		final Event event2 = new Event(2L);
+		final Event event3 = new Event(3L);
+		map.addAll(event1, event2, event3);
+		assertEquals(3, map.size());
+
+		// Update all to non-null timestamps
+		assertDoesNotThrow(() ->
+		{
+			map.update(event1, e -> e.timestamp = Instant.parse("2025-06-15T10:30:00Z"));
+			map.update(event2, e -> e.timestamp = Instant.parse("2025-06-15T11:30:00Z"));
+			map.update(event3, e -> e.timestamp = Instant.parse("2025-06-15T12:30:00Z"));
+			map.update(event1, e -> {});
+			map.update(event2, e -> {});
+			map.update(event3, e -> {});
+		});
+
+		assertEquals(3, map.size());
+	}
+
+	@Test
+	void nullToNonNullQueryShouldWork()
+	{
+		final GigaMap<Event> map = GigaMap.New();
+		final EventIndexer indexer = new EventIndexer();
+		map.index().bitmap().ensure(indexer);
+
+		// Add entities with mixed null/non-null timestamps
+		final Event event1 = new Event(1L);
+		final Event event2 = new Event(2L);
+		event2.timestamp = Instant.parse("2025-06-15T10:30:00Z");
+		map.addAll(event1, event2);
+
+		// Query for null timestamps
+		assertEquals(1, map.query(indexer.is((Instant) null)).count());
+
+		// Query for non-null timestamps
+		assertEquals(1, map.query(indexer.is(Instant.parse("2025-06-15T10:30:00Z"))).count());
+
+		// Update null to non-null
+		map.update(event1, e -> e.timestamp = Instant.parse("2025-06-15T11:30:00Z"));
+
+		// Verify queries after update
+		assertEquals(0, map.query(indexer.is((Instant) null)).count());
+		assertEquals(1, map.query(indexer.is((Instant) null).or(indexer.is(Instant.parse("2025-06-15T10:30:00Z")))).count());
+	}
+}