diff --git a/src/CSPBuilder.php b/src/CSPBuilder.php
index 798d95d..26867b2 100644
--- a/src/CSPBuilder.php
+++ b/src/CSPBuilder.php
@@ -424,6 +424,10 @@ protected function compileSubgroup($directive, $policies = null)
             return $directive." 'none'; ";
         }
         $ret = $directive.' ';
+        if ($directive === 'plugin-types') {
+            // Expects MIME types, not URLs
+            return $ret . \implode(' ', $policies['allow']);
+        }
         if (!empty($policies['self'])) {
             $ret .= "'self' ";
         }