diff --git a/includes/form.inc b/includes/form.inc
index 5a212e6..d6fc72b 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -2571,7 +2571,7 @@ function form_type_select_value($element, $input = FALSE) {
  *   for this element. Return nothing to use the default.
  */
 function form_type_textarea_value($element, $input = FALSE) {
-  if ($input !== FALSE) {
+  if ($input !== FALSE && $input !== NULL) {
     // This should be a string, but allow other scalars since they might be
     // valid input in programmatic form submissions.
     return is_scalar($input) ? (string) $input : '';
diff --git a/modules/simpletest/tests/form.test b/modules/simpletest/tests/form.test
index 0bf6c8c..ad303c5 100644
--- a/modules/simpletest/tests/form.test
+++ b/modules/simpletest/tests/form.test
@@ -2099,3 +2099,36 @@ class HTMLIdTestCase extends DrupalWebTestCase {
     $this->assertNoDuplicateIds('There are no duplicate IDs');
   }
 }
+
+/**
+ * Tests for form textarea.
+ */
+class FormTextareaTestCase extends DrupalUnitTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Form textarea',
+      'description' => 'Tests form textarea related functions.',
+      'group' => 'Form API',
+    );
+  }
+
+  /**
+   * Tests that textarea value is properly set.
+   */
+  public function testValueCallback() {
+    $element = array();
+    $form_state = array();
+    $test_cases = array(
+      array(NULL, FALSE),
+      array(NULL, NULL),
+      array('', array('test')),
+      array('test', 'test'),
+      array('123', 123),
+    );
+    foreach ($test_cases as $test_case) {
+      list($expected, $input) = $test_case;
+      $this->assertIdentical($expected, form_type_textarea_value($element, $input, $form_state));
+    }
+  }
+}