Index: library/Zend/Form/Element/Multi.php
===================================================================
--- library/Zend/Form/Element/Multi.php	(revision 12418)
+++ library/Zend/Form/Element/Multi.php	(working copy)
@@ -245,11 +245,23 @@
     {
         if ($this->registerInArrayValidator()) {
             if (!$this->getValidator('InArray')) {
-                $options = $this->getMultiOptions();
+                $multiOptions = $this->getMultiOptions();
+                $options      = array();
+                
+                foreach ($multiOptions as $opt_value => $opt_label) {
+                    // optgroup instead of option label
+                    if (is_array($opt_label)) {
+                        $options = array_merge($options, array_keys($opt_label));
+                    }
+                    else {
+                        $options[] = $opt_value;
+                    }
+                }
+                
                 $this->addValidator(
                     'InArray',
                     true,
-                    array(array_keys($options))
+                    array($options)
                 );
             }
         }
Index: tests/Zend/Form/Element/SelectTest.php
===================================================================
--- tests/Zend/Form/Element/SelectTest.php	(revision 12418)
+++ tests/Zend/Form/Element/SelectTest.php	(working copy)
@@ -169,6 +169,49 @@
         $this->assertNotContains('unused', $html, $html);
         $this->assertContains('bar', $html, $html);
     }
+    
+    /**
+     * Test isValid() on select elements without optgroups. This
+     * ensures fixing ZF-3985 doesn't break existing functionality.
+     * 
+     * @see ZF-3985
+     */
+    public function testIsValidWithPlainOptions()
+    {
+        // test both syntaxes for setting plain options
+        $this->element->setMultiOptions(array(
+            array('key' => '1', 'value' => 'Web Developer'),
+            '2' => 'Software Engineer',
+        ));
+        
+        $this->assertTrue($this->element->isValid('1'));
+        $this->assertTrue($this->element->isValid('2'));
+        $this->assertFalse($this->element->isValid('3'));
+        $this->assertFalse($this->element->isValid('Web Developer'));
+    }
+    
+    /**
+     * @group ZF-3985
+     */
+    public function testIsValidWithOptionGroups()
+    {
+        // test optgroup and both syntaxes for setting plain options
+        $this->element->setMultiOptions(array(
+            'Technology' => array(
+                '1' => 'Web Developer',
+                '2' => 'Software Engineer',
+            ),
+            array('key' => '3', 'value' => 'Trainee'),
+            '4' => 'Intern',
+        ));
+        
+        $this->assertTrue($this->element->isValid('1'));
+        $this->assertTrue($this->element->isValid('3'));
+        $this->assertTrue($this->element->isValid('4'));
+        $this->assertFalse($this->element->isValid('5'));
+        $this->assertFalse($this->element->isValid('Technology'));
+        $this->assertFalse($this->element->isValid('Web Developer'));
+    }
 
     /**
      * Used by test methods susceptible to ZF-2794, marks a test as incomplete