diff --git a/src/CSS.php b/src/CSS.php
index ece9c6d..1528666 100644
--- a/src/CSS.php
+++ b/src/CSS.php
@@ -308,6 +308,7 @@ class CSS extends Minify
             $this->extractStrings();
             $this->stripComments();
             $this->extractMath();
+            $this->extractCustomProperties();
             $css = $this->replace($css);
 
             $css = $this->stripWhitespace($css);
@@ -721,6 +722,25 @@ class CSS extends Minify
         );
     }
 
+    /**
+     * Replace custom properties, whose values may be used in scenarios where
+     * we wouldn't want them to be minified (e.g. inside calc)
+     */
+    protected function extractCustomProperties()
+    {
+        // PHP only supports $this inside anonymous functions since 5.4
+        $minifier = $this;
+        $this->registerPattern(
+            '/(?<=^|[;}])(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
+            function ($match) use ($minifier) {
+                $placeholder = '--custom-'. count($minifier->extracted) . ':0';
+                $minifier->extracted[$placeholder] = $match[1] .':'. trim($match[2]);
+                return $placeholder;
+
+            }
+        );
+    }
+
     /**
      * Check if file is small enough to be imported.
      *
diff --git a/tests/css/CSSTest.php b/tests/css/CSSTest.php
index f8e61ae..89f3868 100644
--- a/tests/css/CSSTest.php
+++ b/tests/css/CSSTest.php
@@ -806,6 +806,13 @@ body{font-family:sans-serif}',
             'clamp(2.5rem, 1rem + 4vw, 4rem)',
         );
 
+        // https://github.com/matthiasmullie/minify/issues/342
+        $tests[] = array(
+            '--headlineFontSize: 16px + var(--multiplicator);
+font-size: calc(var(--headlineFontSize));',
+            '--headlineFontSize:16px + var(--multiplicator);font-size:calc(var(--headlineFontSize));',
+        );
+
         return $tests;
     }