diff -u b/src/PathProcessor.php b/src/PathProcessor.php
--- b/src/PathProcessor.php
+++ b/src/PathProcessor.php
@@ -121,20 +121,23 @@
       return $path;
     }
     $original_path = $path;
-    $subpath = [];
-    $max_depth = $this->getMaxDepth();
-    $i = 0;
-    while (($path_array = explode('/', ltrim($path, '/'))) && ($max_depth === 0 || $i < $max_depth)) {
-      $i++;
-      $subpath[] = array_pop($path_array);
-      if (empty($path_array)) {
-        break;
-      }
-      $path = '/' . implode('/', $path_array);
-      $processed_path = $this->pathProcessor->processOutbound($path, $options, $request);
-      if ($processed_path !== $path) {
-        $path = $processed_path . '/' . implode('/', array_reverse($subpath));
-        return $path;
+    if ($valid_path = $this->isValidPath($path)) {
+      $subpath = [];
+      $max_depth = $this->getMaxDepth();
+      $i = 0;
+      while (($path_array = explode('/', ltrim($path, '/'))) && ($max_depth === 0 || $i < $max_depth)) {
+        $i++;
+        $subpath[] = array_pop($path_array);
+        if (empty($path_array)) {
+          break;
+        }
+        $path = '/' . implode('/', $path_array);
+
+        $processed_path = $this->pathProcessor->processOutbound($path, $options, $request);
+        if ($processed_path !== $path) {
+          $path = $processed_path . '/' . implode('/', array_reverse($subpath));
+          return $path;
+        }
       }
     }
 
@@ -179,12 +182,9 @@
 
     // Ignore whitelisted paths and return false
     $blacklist = \Drupal::config('subpathauto.settings')->get('blacklist');
-    $patterns = explode('\n', $blacklist);
-    if ($patterns) {
-      foreach ($patterns as $pattern) {
-        if (\Drupal::service('path.matcher')->matchPath($path, $pattern)) {
-          return FALSE;
-        }
+    if ($blacklist) {
+      if (\Drupal::service('path.matcher')->matchPath($path, $blacklist)) {
+        return FALSE;
       }
     }