diff --git a/apisix/plugins/proxy-cache/util.lua b/apisix/plugins/proxy-cache/util.lua
index f20d2fc21275..26c6e814b353 100644
--- a/apisix/plugins/proxy-cache/util.lua
+++ b/apisix/plugins/proxy-cache/util.lua
@@ -37,7 +37,7 @@ function _M.generate_complex_value(data, ctx)
         core.log.info("proxy-cache complex value index-", i, ": ", value)
 
         if string.byte(value, 1, 1) == string.byte('$') then
-            tmp[i] = ctx.var[string.sub(value, 2)]
+            tmp[i] = ctx.var[string.sub(value, 2)] or ""
         else
             tmp[i] = value
         end
diff --git a/docs/en/latest/plugins/proxy-cache.md b/docs/en/latest/plugins/proxy-cache.md
index a21d870899c0..0539c07116bc 100644
--- a/docs/en/latest/plugins/proxy-cache.md
+++ b/docs/en/latest/plugins/proxy-cache.md
@@ -53,6 +53,10 @@ The cache expiration time cannot be configured dynamically. It can only be set b
 
 If the Upstream service is not available and APISIX returns a 502 or 504 status code, it will be cached for 10s.
 
+Variables (start with `$`) can be specified in `cache_key`, `cache_bypass` and `no_cache`. It's worth mentioning that the variable value will be an empty string if it doesn't exist.
+
+You can also combine a number of variables and strings (constants), by writing them into an array, eventually, variables will be parsed and stitched together with strings.
+
 :::
 
 ## Enabling the Plugin
diff --git a/docs/zh/latest/plugins/proxy-cache.md b/docs/zh/latest/plugins/proxy-cache.md
index b2db7c7246de..b22e4eb1ce4c 100644
--- a/docs/zh/latest/plugins/proxy-cache.md
+++ b/docs/zh/latest/plugins/proxy-cache.md
@@ -45,7 +45,7 @@ title: proxy-cache
 | no_cache           | array[string]  | 可选   |                           |                                                                                 | 是否缓存数据，可以使用变量，需要注意当此参数的值不为空或非'0'时将不会缓存数据                                                      |
 | cache_ttl          | integer        | 可选   | 300 秒                    |                                                                                 | 当选项 cache_control 未开启或开启以后服务端没有返回缓存控制头时，提供的默认缓存时间    |
 
-注：变量以$开头，也可以使用变量和字符串的结合，但是需要以数组的形式分开写，最终变量被解析后会和字符串拼接在一起。
+注：变量以$开头，不存在时等价于空字符串。也可以使用变量和字符串的结合，但是需要以数组的形式分开写，最终变量被解析后会和字符串拼接在一起。
 
 在 `conf/config.yaml` 文件中的配置示例：
 
diff --git a/t/plugin/proxy-cache/disk.t b/t/plugin/proxy-cache/disk.t
index 954f967f725d..98e6e1d8b735 100644
--- a/t/plugin/proxy-cache/disk.t
+++ b/t/plugin/proxy-cache/disk.t
@@ -707,3 +707,53 @@ GET /t
 --- error_code: 400
 --- response_body eval
 qr/failed to check the configuration of plugin proxy-cache err/
+
+
+
+=== TEST 28: nil vars for cache_key
+--- config
+       location /t {
+           content_by_lua_block {
+               local t = require("lib.test_admin").test
+               local code, body = t('/apisix/admin/routes/1',
+                    ngx.HTTP_PUT,
+                    [[{
+                        "plugins": {
+                            "proxy-cache": {
+                               "cache_key": ["$arg_foo", "$arg_bar", "$arg_baz"],
+                               "cache_zone": "disk_cache_one",
+                               "cache_bypass": ["$arg_bypass"],
+                               "cache_method": ["GET"],
+                               "cache_http_status": [200],
+                               "hide_cache_headers": true,
+                               "no_cache": ["$arg_no_cache"]
+                            }
+                        },
+                        "upstream": {
+                            "nodes": {
+                                "127.0.0.1:1986": 1
+                            },
+                            "type": "roundrobin"
+                        },
+                        "uri": "/hello"
+                   }]]
+                   )
+
+               if code >= 300 then
+                   ngx.status = code
+               end
+               ngx.say(body)
+           }
+       }
+--- request
+GET /t
+--- response_body
+passed
+
+
+
+=== TEST 29: hit route with nil vars in cache_key
+--- request
+GET /hello?bar=a
+--- response_body chop
+hello world!