diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/FileRepositoryBuilderTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/FileRepositoryBuilderTest.java
index 80e0f1a92..aed48aa5f 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/FileRepositoryBuilderTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/FileRepositoryBuilderTest.java
@@ -44,10 +44,14 @@
 package org.eclipse.jgit.storage.file;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.fail;
 
 import java.io.File;
+import java.io.IOException;
 
 import org.eclipse.jgit.junit.LocalDiskRepositoryTestCase;
+import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.util.FileUtils;
 import org.junit.Test;
 
@@ -61,4 +65,47 @@ public class FileRepositoryBuilderTest extends LocalDiskRepositoryTestCase {
 		assertEquals(r.getDirectory(), new FileRepositoryBuilder()
 				.findGitDir(d).getGitDir());
 	}
+
+	@Test
+	public void emptyRepositoryFormatVersion() throws Exception {
+		FileRepository r = createWorkRepository();
+		FileBasedConfig config = r.getConfig();
+		config.setString(ConfigConstants.CONFIG_CORE_SECTION, null,
+				ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, "");
+		config.save();
+
+		new FileRepository(r.getDirectory());
+	}
+
+	@Test
+	public void invalidRepositoryFormatVersion() throws Exception {
+		FileRepository r = createWorkRepository();
+		FileBasedConfig config = r.getConfig();
+		config.setString(ConfigConstants.CONFIG_CORE_SECTION, null,
+				ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, "notanumber");
+		config.save();
+
+		try {
+			new FileRepository(r.getDirectory());
+			fail("IllegalArgumentException not thrown");
+		} catch (IllegalArgumentException e) {
+			assertNotNull(e.getMessage());
+		}
+	}
+
+	@Test
+	public void unknownRepositoryFormatVersion() throws Exception {
+		FileRepository r = createWorkRepository();
+		FileBasedConfig config = r.getConfig();
+		config.setLong(ConfigConstants.CONFIG_CORE_SECTION, null,
+				ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, 1);
+		config.save();
+
+		try {
+			new FileRepository(r.getDirectory());
+			fail("IOException not thrown");
+		} catch (IOException e) {
+			assertNotNull(e.getMessage());
+		}
+	}
 }
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/T0003_BasicTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/T0003_BasicTest.java
index 37d28d2ab..2a9bb7a56 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/T0003_BasicTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/storage/file/T0003_BasicTest.java
@@ -370,9 +370,8 @@ public class T0003_BasicTest extends SampleDataRepositoryTestCase {
 		try {
 			new FileRepository(db.getDirectory());
 			fail("incorrectly opened a bad repository");
-		} catch (IOException ioe) {
-			assertTrue(ioe.getMessage().indexOf("format") > 0);
-			assertTrue(ioe.getMessage().indexOf(badvers) > 0);
+		} catch (IllegalArgumentException ioe) {
+			assertNotNull(ioe.getMessage());
 		}
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java
index f1a8367ee..fa87b9f26 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java
@@ -181,14 +181,13 @@ public class FileRepository extends Repository {
 				getFS());
 
 		if (objectDatabase.exists()) {
-			final String repositoryFormatVersion = getConfig().getString(
+			final long repositoryFormatVersion = getConfig().getLong(
 					ConfigConstants.CONFIG_CORE_SECTION, null,
-					ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION);
-			if (!"0".equals(repositoryFormatVersion)) {
+					ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, 0);
+			if (repositoryFormatVersion > 0)
 				throw new IOException(MessageFormat.format(
 						JGitText.get().unknownRepositoryFormat2,
-						repositoryFormatVersion));
-			}
+						Long.valueOf(repositoryFormatVersion)));
 		}
 
 		if (!isBare())