diff --git a/searx/engines/duckduckgo.py b/searx/engines/duckduckgo.py
index 3ce629eec..5ae456b04 100644
--- a/searx/engines/duckduckgo.py
+++ b/searx/engines/duckduckgo.py
@@ -61,17 +61,7 @@ form_data = {'v': 'l', 'api': 'd.js', 'o': 'json'}
 
 
 def cache_vqd(query, value):
-    """Caches a ``vqd`` value from a query.
-
-    The vqd value depends on the query string and is needed for the follow up
-    pages or the images loaded by a XMLHttpRequest:
-
-    - DuckDuckGo Web: ``https://links.duckduckgo.com/d.js?q=...&vqd=...``
-    - DuckDuckGo Images: ``https://duckduckgo.com/i.js??q=...&vqd=...``
-    - DuckDuckGo Videos: ``https://duckduckgo.com/v.js??q=...&vqd=...``
-    - DuckDuckGo News: ``https://duckduckgo.com/news.js??q=...&vqd=...``
-
-    """
+    """Caches a ``vqd`` value from a query."""
     c = redisdb.client()
     if c:
         logger.debug("cache vqd value: %s", value)
@@ -84,13 +74,43 @@ def get_vqd(query):
     (:py:obj:`cache_vqd`) the query is sent to DDG to get a vqd value from the
     response.
 
+    .. hint::
+
+       If an empty string is returned there are no results for the ``query`` and
+       therefore no ``vqd`` value.
+
+    DDG's bot detection is sensitive to the ``vqd`` value.  For some search terms
+    (such as extremely long search terms that are often sent by bots), no ``vqd``
+    value can be determined.
+
+    If SearXNG cannot determine a ``vqd`` value, then no request should go out
+    to DDG:
+
+        A request with a wrong ``vqd`` value leads to DDG temporarily putting
+        SearXNG's IP on a block list.
+
+        Requests from IPs in this block list run into timeouts.
+
+    Not sure, but it seems the block list is a sliding window: to get my IP rid
+    from the bot list I had to cool down my IP for 1h (send no requests from
+    that IP to DDG).
+
+    TL;DR; the ``vqd`` value is needed to pass DDG's bot protection and is used
+    by all request to DDG:
+
+    - DuckDuckGo Lite: ``https://lite.duckduckgo.com/lite`` (POST form data)
+    - DuckDuckGo Web: ``https://links.duckduckgo.com/d.js?q=...&vqd=...``
+    - DuckDuckGo Images: ``https://duckduckgo.com/i.js??q=...&vqd=...``
+    - DuckDuckGo Videos: ``https://duckduckgo.com/v.js??q=...&vqd=...``
+    - DuckDuckGo News: ``https://duckduckgo.com/news.js??q=...&vqd=...``
+
     """
-    value = None
+    value = ''
     c = redisdb.client()
     if c:
         key = 'SearXNG_ddg_vqd' + redislib.secret_hash(query)
         value = c.get(key)
-        if value:
+        if value or value == b'':
             value = value.decode('utf-8')
             logger.debug("re-use cached vqd value: %s", value)
             return value
@@ -102,9 +122,11 @@ def get_vqd(query):
     if value:
         value = value[0]
     else:
-        # some search terms do not have results and therefore no vqd value
+        # Some search terms do not have results and therefore no vqd value.  If
+        # no vqd value can be determined for the search term, an empty string is
+        # chached.
         value = ''
-    logger.debug("new vqd value: %s", value)
+    logger.debug("new vqd value: '%s'", value)
     cache_vqd(query, value)
     return value
 
@@ -204,6 +226,13 @@ ddg_lang_map = {
 
 def request(query, params):
 
+    # request needs a vqd argument
+    vqd = get_vqd(query)
+    if not vqd:
+        # some search terms do not have results and therefore no vqd value
+        params['url'] = None
+        return params
+
     # quote ddg bangs
     query_parts = []
     # for val in re.split(r'(\s+)', query):
@@ -227,6 +256,7 @@ def request(query, params):
     # link again and again ..
 
     params['headers']['Content-Type'] = 'application/x-www-form-urlencoded'
+    params['data']['vqd'] = vqd
 
     # initial page does not have an offset
     if params['pageno'] == 2:
@@ -248,9 +278,6 @@ def request(query, params):
         params['data']['api'] = form_data.get('api', 'd.js')
         params['data']['nextParams'] = form_data.get('nextParams', '')
         params['data']['v'] = form_data.get('v', 'l')
-
-        # request needs a vqd argument
-        params['data']['vqd'] = get_vqd(query)
         params['headers']['Referer'] = 'https://lite.duckduckgo.com/'
 
     params['data']['kl'] = eng_region
diff --git a/searx/engines/duckduckgo_extra.py b/searx/engines/duckduckgo_extra.py
index 7e3a3282d..25692add7 100644
--- a/searx/engines/duckduckgo_extra.py
+++ b/searx/engines/duckduckgo_extra.py
@@ -48,6 +48,13 @@ search_path_map = {'images': 'i', 'videos': 'v', 'news': 'news'}
 
 def request(query, params):
 
+    # request needs a vqd argument
+    vqd = get_vqd(query)
+    if not vqd:
+        # some search terms do not have results and therefore no vqd value
+        params['url'] = None
+        return params
+
     eng_region = traits.get_region(params['searxng_locale'], traits.all_locale)
     eng_lang = get_ddg_lang(traits, params['searxng_locale'])
 
@@ -57,7 +64,7 @@ def request(query, params):
         # 'u': 'bing',
         'l': eng_region,
         'f': ',,,,,',
-        'vqd': get_vqd(query),
+        'vqd': vqd,
     }
 
     if params['pageno'] > 1: