diff --git includes/form.inc includes/form.inc
index 9018931..bbb5a9b 100644
--- includes/form.inc
+++ includes/form.inc
@@ -1952,7 +1952,7 @@ function _form_builder_handle_input_element($form_id, &$element, &$form_state) {
       // Avoid image buttons (which come with garbage value), so we only get value
       // for the button actually clicked.
       if (!isset($element['#value']) && empty($element['#has_garbage_value'])) {
-        $element['#value'] = isset($element['#default_value']) ? $element['#default_value'] : '';
+        $element['#value'] = array_key_exists('#default_value', $element) ? $element['#default_value'] : '';
       }
     }
   }
diff --git modules/field/modules/options/options.module modules/field/modules/options/options.module
index abff986..e83b468 100644
--- modules/field/modules/options/options.module
+++ modules/field/modules/options/options.module
@@ -103,10 +103,21 @@ function options_field_widget_form(&$form, &$form_state, $field, $instance, $lan
         reset($options);
         $default_value = array(key($options));
       }
+      // Radio buttons need a scalar value.
+      // If there are no default values, NULL needs to be passed so that the
+      // appropriate validation is triggered.
+      if (!$multiple) {
+        if (!empty($default_value)) {
+          $default_value = reset($default_value);
+        }
+        else {
+          $default_value = NULL;
+        }
+      }
+      
       $element += array(
         '#type' => $multiple ? 'checkboxes' : 'radios',
-        // Radio buttons need a scalar value.
-        '#default_value' => $multiple ? $default_value : reset($default_value),
+        '#default_value' => $default_value,
         '#options' => $options,
       );
       break;
diff --git modules/system/system.module modules/system/system.module
index d6d822a..70b8e44 100644
--- modules/system/system.module
+++ modules/system/system.module
@@ -392,6 +392,11 @@ function system_element_info() {
   );
   $types['radios'] = array(
     '#input' => TRUE,
+    // If no #default_value is specified and a user agent submits no value for
+    // #type 'radios', then _form_builder_handle_input_element() would assign an
+    // empty string as #value, which in turn would trigger an "illegal choice"
+    // error in _form_validate().
+    '#default_value' => NULL,
     '#process' => array('form_process_radios'),
     '#theme_wrappers' => array('radios'),
     '#pre_render' => array('form_pre_render_conditional_form_element'),