variable '--van-gray-6' is undefined and used without a fallback

[FANZHETWO]

When I start webpack compilation, Terminal prompts the error shown in the following image. An error means that CSS variables are not defined. How can I deal with this problem

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:258) postcss-custom-properties: variable '--van-gray-5' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/loading/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1170) postcss-custom-properties: variable '--van-padding-xs' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/loading/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1204) postcss-custom-properties: variable '--van-gray-6' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/loading/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1240) postcss-custom-properties: variable '--van-font-size-md' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/loading/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1425) postcss-custom-properties: variable '--van-padding-xs' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:465) postcss-custom-properties: variable '--van-white' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:516) postcss-custom-properties: variable '--van-animation-duration-base' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1736) postcss-custom-properties: variable '--van-animation-timing-function-enter' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:1948) postcss-custom-properties: variable '--van-animation-timing-function-leave' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):
Warning

(1:2515) postcss-custom-properties: variable '--van-gray-5' is undefined and used without a fallback

 warning  in ./node_modules/vant/es/popup/index.css

Module Warning (from ./node_modules/postcss-loader/dist/cjs.js):

[jtomaszewski]

In our case, we would like to get such errors (when using an undefined variable), but we don't get them... what version of this plugin are you using btw?