diff --git a/src/org/mozilla/javascript/commonjs/module/provider/UrlModuleSourceProvider.java b/src/org/mozilla/javascript/commonjs/module/provider/UrlModuleSourceProvider.java
index 2df6eeb347..b1b6ff2f17 100644
--- a/src/org/mozilla/javascript/commonjs/module/provider/UrlModuleSourceProvider.java
+++ b/src/org/mozilla/javascript/commonjs/module/provider/UrlModuleSourceProvider.java
@@ -130,7 +130,7 @@ protected ModuleSource loadFromUri(URI uri, URI base, Object validator)
     protected ModuleSource loadFromActualUri(URI uri, URI base, Object validator)
     throws IOException
     {
-        final URL url = uri.toURL();
+        final URL url = new URL(base.toURL(), uri.toString());
         final long request_time = System.currentTimeMillis();
         final URLConnection urlConnection = openUrlConnection(url);
         final URLValidator applicableValidator;
diff --git a/testsrc/org/mozilla/javascript/tests/commonjs/module/RequireJarTest.java b/testsrc/org/mozilla/javascript/tests/commonjs/module/RequireJarTest.java
new file mode 100644
index 0000000000..c4d45efaaa
--- /dev/null
+++ b/testsrc/org/mozilla/javascript/tests/commonjs/module/RequireJarTest.java
@@ -0,0 +1,113 @@
+package org.mozilla.javascript.tests.commonjs.module;
+
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.util.Collections;
+
+import org.mozilla.javascript.Context;
+import org.mozilla.javascript.Scriptable;
+import org.mozilla.javascript.ScriptableObject;
+import org.mozilla.javascript.commonjs.module.Require;
+import org.mozilla.javascript.commonjs.module.provider.StrongCachingModuleScriptProvider;
+import org.mozilla.javascript.commonjs.module.provider.UrlModuleSourceProvider;
+
+/**
+ * @author Attila Szegedi
+ * @version $Id: RequireTest.java,v 1.1 2011/04/07 22:24:37 hannes%helma.at Exp $
+ */
+public class RequireJarTest extends RequireTest
+{
+    public void testSandboxed() throws Exception
+    {
+        final Context cx = createContext();
+        final Require require = getSandboxedRequire(cx);
+        require.requireMain(cx, "testSandboxed");
+        // Also, test idempotent double-require of same main:
+        require.requireMain(cx, "testSandboxed");
+        // Also, test failed require of different main:
+        try {
+            require.requireMain(cx, "blah");
+            fail();
+        }
+        catch(IllegalStateException e) {
+            // Expected, success
+        }
+    }
+
+    private Context createContext()
+    {
+        final Context cx = Context.enter(); 
+        cx.setOptimizationLevel(-1);
+        return cx;
+    }
+
+    public void testNonSandboxed() throws Exception
+    {
+        final Context cx = createContext();
+        final Scriptable scope = cx.initStandardObjects();
+        final Require require = getSandboxedRequire(cx, scope, false);
+        final String jsFile = getClass().getResource("testNonSandboxed.js").toExternalForm();
+        ScriptableObject.putProperty(scope, "moduleUri", jsFile);
+        require.requireMain(cx, "testNonSandboxed");
+    }
+
+    public void testVariousUsageErrors() throws Exception {
+        testWithSandboxedRequire("testNoArgsRequire");
+    }
+
+    public void testRelativeId() throws Exception {
+        final Context cx = createContext();
+        final Scriptable scope = cx.initStandardObjects();
+        final Require require = getSandboxedRequire(cx, scope, false);
+        require.install(scope);
+        cx.evaluateReader(scope, getReader("testRelativeId.js"), 
+                "testRelativeId.js", 1, null);
+    }
+
+    public void testSetMainForAlreadyLoadedModule() throws Exception {
+        final Context cx = createContext();
+        final Scriptable scope = cx.initStandardObjects();
+        final Require require = getSandboxedRequire(cx, scope, false);
+        require.install(scope);
+        cx.evaluateReader(scope, getReader("testSetMainForAlreadyLoadedModule.js"), 
+                "testSetMainForAlreadyLoadedModule.js", 1, null);
+        try {
+            require.requireMain(cx, "assert");
+            fail();
+        }
+        catch(IllegalStateException e) {
+            assertEquals(e.getMessage(), "Attempt to set main module after it was loaded");
+        }
+    }
+    
+    private Reader getReader(String name) {
+        return new InputStreamReader(getClass().getResourceAsStream(name));
+    }
+    
+    private void testWithSandboxedRequire(String moduleId) throws Exception {
+        final Context cx = createContext();
+        getSandboxedRequire(cx).requireMain(cx, moduleId);
+    }
+
+    private Require getSandboxedRequire(final Context cx)
+    throws URISyntaxException {
+        return getSandboxedRequire(cx, cx.initStandardObjects(), true);
+    }
+    
+    private Require getSandboxedRequire(Context cx, Scriptable scope, boolean sandboxed)
+            throws URISyntaxException
+    {
+        return new Require(cx, cx.initStandardObjects(), 
+                new StrongCachingModuleScriptProvider(
+                        new UrlModuleSourceProvider(Collections.singleton(
+                                getDirectory()), null)), null, null, true);
+    }
+
+    private URI getDirectory() throws URISyntaxException {
+        final String jarFileLoc = getClass().getResource("modules.jar").toURI().toString();
+        String jarParent = "jar:" + jarFileLoc + "!/";
+        return new URI(jarParent);
+    }
+}
diff --git a/testsrc/org/mozilla/javascript/tests/commonjs/module/modules.jar b/testsrc/org/mozilla/javascript/tests/commonjs/module/modules.jar
new file mode 100644
index 0000000000..0dab4857e3
Binary files /dev/null and b/testsrc/org/mozilla/javascript/tests/commonjs/module/modules.jar differ