diff --git a/src/CSS.php b/src/CSS.php
index 44c61a1..3ba9e55 100644
--- a/src/CSS.php
+++ b/src/CSS.php
@@ -722,7 +722,7 @@ protected function extractMath()
             // and since we've captured more code than required, we may have some leftover
             // calc() in here too - go recursive on the remaining but of code to go figure
             // that out and extract what is needed
-            $rest = str_replace($function.$expr, '', $match[0]);
+            $rest = $minifier->str_replace_first($function.$expr, '', $match[0]);
             $rest = preg_replace_callback($pattern, $callback, $rest);
 
             return $placeholder.$rest;
diff --git a/src/Minify.php b/src/Minify.php
index 736ff11..2ff0c16 100644
--- a/src/Minify.php
+++ b/src/Minify.php
@@ -494,4 +494,12 @@ protected function writeToFile($handler, $content, $path = '')
             throw new IOException('The file "'.$path.'" could not be written to. Check your disk space and file permissions.');
         }
     }
+
+    protected static function str_replace_first($search, $replace, $subject) {
+        $pos = strpos($subject, $search);
+        if ($pos !== false) {
+            return substr_replace($subject, $replace, $pos, strlen($search));
+        }
+        return $subject;
+    }
 }
diff --git a/tests/css/CSSTest.php b/tests/css/CSSTest.php
index 634f88b..a9e05eb 100644
--- a/tests/css/CSSTest.php
+++ b/tests/css/CSSTest.php
@@ -849,6 +849,12 @@ public function dataProvider()
             '.stk-block-columns>.stk-block-content{--stk-column-gap:0px;column-gap:var(--stk-column-gap,0)}'
         );
 
+        // https://github.com/matthiasmullie/minify/issues/395
+        $tests[] = array(
+            'background-position: right 0.8em bottom calc(50% - 5px), right 0.8em top calc(50% - 5px);',
+            'background-position:right .8em bottom calc(50% - 5px),right .8em top calc(50% - 5px);'
+        );
+
         return $tests;
     }