diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfigTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfigTest.java
new file mode 100644
index 000000000..e612061ab
--- /dev/null
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfigTest.java
@@ -0,0 +1,88 @@
+/*
+ * Copyright (C) 2016, Philipp Marx <philippmarx@gmx.de> and
+ * other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v1.0 which accompanies this
+ * distribution, is reproduced below, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from this
+ * software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.internal.storage.dfs;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+
+import org.eclipse.jgit.internal.JGitText;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class DfsBlockCacheConfigTest {
+
+	@Rule
+	public ExpectedException thrown = ExpectedException.none();
+
+	@Test
+	public void blockSizeNotPowerOfTwoExpectsException() {
+		thrown.expect(IllegalArgumentException.class);
+		thrown.expectMessage(is(JGitText.get().blockSizeNotPowerOf2));
+
+		new DfsBlockCacheConfig().setBlockSize(1000);
+	}
+
+	@Test
+	@SuppressWarnings("boxing")
+	public void negativeBlockSizeIsConvertedToDefault() {
+		DfsBlockCacheConfig config = new DfsBlockCacheConfig();
+		config.setBlockSize(-1);
+
+		assertThat(config.getBlockSize(), is(512));
+	}
+
+	@Test
+	@SuppressWarnings("boxing")
+	public void tooSmallBlockSizeIsConvertedToDefault() {
+		DfsBlockCacheConfig config = new DfsBlockCacheConfig();
+		config.setBlockSize(10);
+
+		assertThat(config.getBlockSize(), is(512));
+	}
+
+	@Test
+	@SuppressWarnings("boxing")
+	public void validBlockSize() {
+		DfsBlockCacheConfig config = new DfsBlockCacheConfig();
+		config.setBlockSize(65536);
+
+		assertThat(config.getBlockSize(), is(65536));
+	}
+}
diff --git a/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties b/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
index 399436dfa..aa73b4484 100644
--- a/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
+++ b/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
@@ -39,6 +39,7 @@ bitmapsMustBePrepared=Bitmaps must be prepared before they may be written.
 blameNotCommittedYet=Not Committed Yet
 blobNotFound=Blob not found: {0}
 blobNotFoundForPath=Blob not found: {0} for path: {1}
+blockSizeNotPowerOf2=blockSize must be a power of 2
 branchNameInvalid=Branch name {0} is not allowed
 buildingBitmaps=Building bitmaps
 cachedPacksPreventsIndexCreation=Using cached packs prevents index creation
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
index 00d019184..29b319fe6 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
@@ -98,6 +98,7 @@ public class JGitText extends TranslationBundle {
 	/***/ public String blameNotCommittedYet;
 	/***/ public String blobNotFound;
 	/***/ public String blobNotFoundForPath;
+	/***/ public String blockSizeNotPowerOf2;
 	/***/ public String branchNameInvalid;
 	/***/ public String buildingBitmaps;
 	/***/ public String cachedPacksPreventsIndexCreation;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCache.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCache.java
index 764ae1284..05627ed86 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCache.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCache.java
@@ -145,6 +145,8 @@ public final class DfsBlockCache {
 	 * <p>
 	 * If a pack file has a native size, a whole multiple of the native size
 	 * will be used until it matches this size.
+	 * <p>
+	 * The value for blockSize must be a power of 2.
 	 */
 	private final int blockSize;
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfig.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfig.java
index 7e32554c6..089bfa471 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfig.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsBlockCacheConfig.java
@@ -106,10 +106,16 @@ public class DfsBlockCacheConfig {
 	/**
 	 * @param newSize
 	 *            size in bytes of a single window read in from the pack file.
+	 *            The value must be a power of 2.
 	 * @return {@code this}
 	 */
 	public DfsBlockCacheConfig setBlockSize(final int newSize) {
-		blockSize = Math.max(512, newSize);
+		int size = Math.max(512, newSize);
+		if ((size & (size - 1)) != 0) {
+			throw new IllegalArgumentException(
+					JGitText.get().blockSizeNotPowerOf2);
+		}
+		blockSize = size;
 		return this;
 	}