diff -u b/core/includes/form.inc b/core/includes/form.inc
--- b/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -2341,8 +2341,9 @@
     // When there's user input (including NULL), return it as the value.
     // However, if NULL is submitted, _form_builder_handle_input_element() will
     // apply the default value, and we want that validated against #options
-    // unless it's NULL or FALSE.
-    if (!isset($input) && isset($element['#default_value']) && $element['#default_value'] !== FALSE) {
+    // unless it's empty. (An empty #default_value, such as NULL or FALSE, can
+    // be used to indicate that no radio button is selected by default.)
+    if (!isset($input) && !empty($element['#default_value'])) {
       $element['#needs_validation'] = TRUE;
     }
     return $input;
@@ -2351,8 +2352,8 @@
     // For default value handling, simply return #default_value. Additionally,
     // for a NULL default value, set #has_garbage_value to prevent
     // _form_builder_handle_input_element() converting the NULL to an empty
-    // string, so that code can distinguish between nothing selected and a
-    // radio button whose value is empty string selected.
+    // string, so that code can distinguish between nothing selected and the
+    // selection of a radio button whose value is an empty string.
     $value = isset($element['#default_value']) ? $element['#default_value'] : NULL;
     if (!isset($value)) {
       $element['#has_garbage_value'] = TRUE;