diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/submodule/SubmoduleAddTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/submodule/SubmoduleAddTest.java
index dee2accf2..4c5a2c138 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/submodule/SubmoduleAddTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/submodule/SubmoduleAddTest.java
@@ -66,6 +66,7 @@ import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.RepositoryTestCase;
 import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.junit.Test;
 
 /**
@@ -212,4 +213,44 @@ public class SubmoduleAddTest extends RepositoryTestCase {
 		assertTrue(status.getAdded().contains(Constants.DOT_GIT_MODULES));
 		assertTrue(status.getAdded().contains(path));
 	}
+
+	@Test
+	public void addSubmoduleWithExistingSubmoduleDefined() throws Exception {
+		String path1 = "sub1";
+		String url1 = "git://server/repo1.git";
+		String path2 = "sub2";
+
+		FileBasedConfig modulesConfig = new FileBasedConfig(new File(
+				db.getWorkTree(), Constants.DOT_GIT_MODULES), db.getFS());
+		modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION,
+				path1, ConfigConstants.CONFIG_KEY_PATH, path1);
+		modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION,
+				path1, ConfigConstants.CONFIG_KEY_URL, url1);
+		modulesConfig.save();
+
+		Git git = new Git(db);
+		writeTrashFile("file.txt", "content");
+		git.add().addFilepattern("file.txt").call();
+		assertNotNull(git.commit().setMessage("create file").call());
+
+		SubmoduleAddCommand command = new SubmoduleAddCommand(db);
+		command.setPath(path2);
+		String url2 = db.getDirectory().toURI().toString();
+		command.setURI(url2);
+		assertNotNull(command.call());
+
+		modulesConfig.load();
+		assertEquals(path1, modulesConfig.getString(
+				ConfigConstants.CONFIG_SUBMODULE_SECTION, path1,
+				ConfigConstants.CONFIG_KEY_PATH));
+		assertEquals(url1, modulesConfig.getString(
+				ConfigConstants.CONFIG_SUBMODULE_SECTION, path1,
+				ConfigConstants.CONFIG_KEY_URL));
+		assertEquals(path2, modulesConfig.getString(
+				ConfigConstants.CONFIG_SUBMODULE_SECTION, path2,
+				ConfigConstants.CONFIG_KEY_PATH));
+		assertEquals(url2, modulesConfig.getString(
+				ConfigConstants.CONFIG_SUBMODULE_SECTION, path2,
+				ConfigConstants.CONFIG_KEY_URL));
+	}
 }
\ No newline at end of file
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleAddCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleAddCommand.java
index e1b293c43..a983134fe 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleAddCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleAddCommand.java
@@ -49,6 +49,7 @@ import java.text.MessageFormat;
 import org.eclipse.jgit.JGitText;
 import org.eclipse.jgit.api.errors.JGitInternalException;
 import org.eclipse.jgit.api.errors.NoFilepatternException;
+import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.NullProgressMonitor;
@@ -177,14 +178,17 @@ public class SubmoduleAddCommand extends
 		// Save path and URL to parent repository's .gitmodules file
 		FileBasedConfig modulesConfig = new FileBasedConfig(new File(
 				repo.getWorkTree(), Constants.DOT_GIT_MODULES), repo.getFS());
-		modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION, path,
-				ConfigConstants.CONFIG_KEY_PATH, path);
-		modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION, path,
-				ConfigConstants.CONFIG_KEY_URL, uri);
 		try {
+			modulesConfig.load();
+			modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION,
+					path, ConfigConstants.CONFIG_KEY_PATH, path);
+			modulesConfig.setString(ConfigConstants.CONFIG_SUBMODULE_SECTION,
+					path, ConfigConstants.CONFIG_KEY_URL, uri);
 			modulesConfig.save();
 		} catch (IOException e) {
 			throw new JGitInternalException(e.getMessage(), e);
+		} catch (ConfigInvalidException e) {
+			throw new JGitInternalException(e.getMessage(), e);
 		}
 
 		AddCommand add = new AddCommand(repo);