PX doesn't work with cssnano and postcss-calc

This is the error I get when using `PX` to ignore a property:

```[14:28:04] JisonLexerError in plugin "gulp-postcss"
Message:
Lexical error on line 1: Unrecognized text.

Erroneous area:
1: 1PX + 0.3em
^..^
Details:
hash: [object Object]
postcssNode: padding:calc(1PX + 0.3em)
fileName: /sass/styles.css
domainEmitter: [object Object]
domain: [object Object]
domainThrown: false

Stack:
JisonLexerError: Lexical error on line 1: Unrecognized text.

Erroneous area:
1: 1PX + 0.3em
^..^
at /sass/styles.css:427:7
at Object.parseError (/node_modules/postcss-calc/dist/parser.js:1164:15)
at Object.lexer_parseError [as parseError] (/node_modules/postcss-calc/dist/parser.js:2297:44)
at Object.lexer_next [as next] (/node_modules/postcss-calc/dist/parser.js:3292:22)
at Object.lexer_fastLex [as fastLex] (/node_modules/postcss-calc/dist/parser.js:3367:18)
at fastLex (/node_modules/postcss-calc/dist/parser.js:1567:27)
at Parser.parse (/node_modules/postcss-calc/dist/parser.js:1641:30)
at /node_modules/postcss-calc/dist/lib/transform.js:30:30
at walk (/node_modules/postcss-value-parser/lib/walk.js:19:7)
at ValueParser.walk (/node_modules/postcss-value-parser/lib/index.js:18:3)
at transformValue (/node_modules/postcss-calc/dist/lib/transform.js:24:50)
at _default (/node_modules/postcss-calc/dist/lib/transform.js:59:100)
at /node_modules/postcss-calc/dist/index.js:25:51
at /node_modules/postcss/lib/container.js:135:18
at Rule.each (/node_modules/postcss/lib/container.js:101:16)
at Rule.walk (/node_modules/postcss/lib/container.js:131:17)
at /node_modules/postcss/lib/container.js:148:24```