When there's no alias defined, link to print*/nid and not print*/node/nid.

print.module

@@ -383,8 +383,8 @@ function print_node_view($node, $view_mode) {
     $query = print_query_string_encode($query_arr, array('q'));
     if (empty($query)) $query = NULL;
 
-    if ($print_html_link_use_alias) {
-      $path = drupal_get_path_alias('node/' . $node->nid);
+    if (($print_html_link_use_alias) && ($alias = drupal_lookup_path('alias', 'node/' . $node->nid))) {
+      $path = $alias;
     }
     else {
       $path = $node->nid;

print_mail/print_mail.module

@@ -163,8 +163,8 @@ function print_mail_node_view($node, $view_mode) {
       $query = print_query_string_encode($query_arr, array('q'));
       if (empty($query)) $query = NULL;
 
-      if ($print_mail_link_use_alias) {
-        $path = drupal_get_path_alias('node/' . $node->nid);
+      if (($print_mail_link_use_alias) && ($alias = drupal_lookup_path('alias', 'node/' . $node->nid))) {
+        $path = $alias;
       }
       else {
         $path = $node->nid;

print_pdf/print_pdf.module

@@ -287,8 +287,8 @@ function print_pdf_node_view($node, $view_mode) {
       $query = print_query_string_encode($query_arr, array('q'));
       if (empty($query)) $query = NULL;
 
-      if ($print_pdf_link_use_alias) {
-        $path = drupal_get_path_alias('node/' . $node->nid);
+      if (($print_pdf_link_use_alias) && ($alias = drupal_lookup_path('alias', 'node/' . $node->nid))) {
+        $path = $alias;
       }
       else {
         $path = $node->nid;