Fixed bug 4775.  The constant AMP was undefined... this was a copy/past from EE code, where AMP exists.
diff --git a/system/libraries/Pagination.php b/system/libraries/Pagination.php
index 4206f4e..5435f42 100644
--- a/system/libraries/Pagination.php
+++ b/system/libraries/Pagination.php
@@ -172,7 +172,7 @@
 		// string. If post, add a trailing slash to the base URL if needed

 		if ($CI->config->item('enable_query_strings') === TRUE OR $this->page_query_string === TRUE)

 		{

-			$this->base_url = rtrim($this->base_url).AMP.$this->query_string_segment.'=';

+			$this->base_url = rtrim($this->base_url).'&'.$this->query_string_segment.'=';

 		}

 		else

 		{