Issue #3376577: PHP notice when enabling functionality - PHP 7

minifyhtml.module

@@ -111,7 +111,8 @@ function minifyhtml_exit() {
     // Make sure to exclude image style images (is this even required
     // anymore??).
     // Make sure the buffer has a length.
-    if (stripos(drupal_get_http_header('content-type'), 'text/html') !== FALSE && !is_file($current_path) && ob_get_length()) {
+    $content_type = drupal_get_http_header('content-type');
+    if (!empty($content_type) && stripos($content_type, 'text/html') !== FALSE && !is_file($current_path) && ob_get_length()) {
 
       // Catch the output buffer. Converted from ob_get_clean().
       $page = ob_get_contents();