diff --git a/src/CSS.php b/src/CSS.php
index 1b354ad..9226df2 100644
--- a/src/CSS.php
+++ b/src/CSS.php
@@ -740,7 +740,7 @@ protected function extractCustomProperties()
         // PHP only supports $this inside anonymous functions since 5.4
         $minifier = $this;
         $this->registerPattern(
-            '/(?<=^|[;}])(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
+            '/(?<=^|[;}])\s*(--[^:;{}"\'\s]+)\s*:([^;{}]+)/m',
             function ($match) use ($minifier) {
                 $placeholder = '--custom-'. count($minifier->extracted) . ':0';
                 $minifier->extracted[$placeholder] = $match[1] .':'. trim($match[2]);
diff --git a/tests/css/CSSTest.php b/tests/css/CSSTest.php
index 6f576f5..4f300b5 100644
--- a/tests/css/CSSTest.php
+++ b/tests/css/CSSTest.php
@@ -832,6 +832,17 @@ public function dataProvider()
             ':root{--some-var:0px}.some-class{margin-left:calc(20px + var(--some-var))}',
         );
 
+        // https://github.com/matthiasmullie/minify/issues/378
+        $tests[] = array(
+            ':root {
+  --some-var: 0px;
+}
+p {
+margin-left: calc(20px + var(--some-var));
+}',
+            ':root{--some-var:0px}p{margin-left:calc(20px + var(--some-var))}'
+        );
+
         return $tests;
     }