Version 4.7.13

system/Email/Incoming/Email.php

@@ -243,7 +243,7 @@ class _Email
             }
 
 			/* We need to convert the text to UTF-8 if it is not already */
-			if ( mb_strtolower( $charset ) != 'utf-8' )
+			if ( mb_strtolower( $charset ) != 'utf-8' and ( in_array( mb_strtolower( $charset ), array_map( 'mb_strtolower', mb_list_encodings() ) ) ) )
 			{
 				$text = mb_convert_encoding( $text, 'UTF-8', mb_strtoupper( $charset ) );
 			}