diff --git a/lib/adminlib.php b/lib/adminlib.php
index 66ad92bbd31..be24d9e2afe 100644
--- a/lib/adminlib.php
+++ b/lib/adminlib.php
@@ -2441,13 +2441,22 @@ class admin_setting_configfile extends admin_setting_configtext {
         '<div class="form-file defaultsnext"><input type="text" size="'.$this->size.'" id="'.$this->get_id().'" name="'.$this->get_full_name().'" value="'.s($data).'" />'.$executable.'</div>',
         $this->description, true, '', $default, $query);
     }
+
     /**
-     * checks if execpatch has been disabled in config.php
+     * Checks if execpatch has been disabled in config.php
      */
     public function write_setting($data) {
         global $CFG;
         if (!empty($CFG->preventexecpath)) {
-            return '';
+            if ($this->get_setting() === null) {
+                // Use default during installation.
+                $data = $this->get_defaultsetting();
+                if ($data === null) {
+                    $data = '';
+                }
+            } else {
+                return '';
+            }
         }
         return parent::write_setting($data);
     }
diff --git a/lib/tests/admintree_test.php b/lib/tests/admintree_test.php
index 82b5d177f54..24677af09ff 100644
--- a/lib/tests/admintree_test.php
+++ b/lib/tests/admintree_test.php
@@ -245,4 +245,57 @@ class core_admintree_testcase extends advanced_testcase {
 
         return $count;
     }
+
+    public function test_preventexecpath() {
+        $this->resetAfterTest();
+
+        set_config('preventexecpath', 0);
+        set_config('execpath', null, 'abc_cde');
+        $this->assertFalse(get_config('abc_cde', 'execpath'));
+        $setting = new admin_setting_configexecutable('abc_cde/execpath', 'some desc', '', '/xx/yy');
+        $setting->write_setting('/oo/pp');
+        $this->assertSame('/oo/pp', get_config('abc_cde', 'execpath'));
+
+        // Prevent changes.
+        set_config('preventexecpath', 1);
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('/oo/pp', get_config('abc_cde', 'execpath'));
+
+        // Use default in install.
+        set_config('execpath', null, 'abc_cde');
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('/xx/yy', get_config('abc_cde', 'execpath'));
+
+        // Use empty value if no default.
+        $setting = new admin_setting_configexecutable('abc_cde/execpath', 'some desc', '', null);
+        set_config('execpath', null, 'abc_cde');
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('', get_config('abc_cde', 'execpath'));
+
+        // This also (most probably incorrectly) affects admin_setting_configfile.
+
+        set_config('preventexecpath', 0);
+        set_config('execpath', null, 'abc_cde');
+        $this->assertFalse(get_config('abc_cde', 'execpath'));
+        $setting = new admin_setting_configfile('abc_cde/execpath', 'some desc', '', '/xx/yy');
+        $setting->write_setting('/oo/pp');
+        $this->assertSame('/oo/pp', get_config('abc_cde', 'execpath'));
+
+        // Prevent changes.
+        set_config('preventexecpath', 1);
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('/oo/pp', get_config('abc_cde', 'execpath'));
+
+        // Use default in install.
+        set_config('execpath', null, 'abc_cde');
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('/xx/yy', get_config('abc_cde', 'execpath'));
+
+        // Use empty value if no default.
+        $setting = new admin_setting_configfile('abc_cde/execpath', 'some desc', '', null);
+        set_config('execpath', null, 'abc_cde');
+        $setting->write_setting('/mm/nn');
+        $this->assertSame('', get_config('abc_cde', 'execpath'));
+
+    }
 }