diff --git a/src/core/build/postcss.config.js b/src/core/build/postcss.config.js
index 3a386d17b..543cc877f 100644
--- a/src/core/build/postcss.config.js
+++ b/src/core/build/postcss.config.js
@@ -37,7 +37,7 @@ const mediaQueryVariables = mapValues(
   // with the smallest screen and gradually add styling for the
   // next bigger screen. This is realized using `min-width` without
   // ever using `max-width`.
-  v => `${Number.parseInt(v) + 1}px`
+  v => `${Number.parseInt(v, 10) + 1}px`
 );
 
 module.exports = {