diff --git a/manage_tags_page.php b/manage_tags_page.php
index 151617924e..b5a9b32d2f 100644
--- a/manage_tags_page.php
+++ b/manage_tags_page.php
@@ -55,7 +55,7 @@
 compress_enable();
 
 $t_can_edit = access_has_global_level( config_get( 'tag_edit_threshold' ) );
-$f_filter = utf8_strtoupper( gpc_get_string( 'filter', config_get( 'default_manage_tag_prefix' ) ) );
+$f_filter = mb_strtoupper( gpc_get_string( 'filter', config_get( 'default_manage_tag_prefix' ) ) );
 $f_page_number = gpc_get_int( 'page_number', 1 );
 
 # Start Index Menu
diff --git a/manage_user_page.php b/manage_user_page.php
index 89aaaa0b0b..0335a235a5 100644
--- a/manage_user_page.php
+++ b/manage_user_page.php
@@ -61,7 +61,7 @@
 $t_lock_image = '<i class="fa fa-lock fa-lg" alt="' . lang_get( 'protected' ) . '" />';
 
 $f_save          = gpc_get_bool( 'save' );
-$f_filter        = utf8_strtoupper( gpc_get_string( 'filter', config_get( 'default_manage_user_prefix' ) ) );
+$f_filter        = mb_strtoupper( gpc_get_string( 'filter', config_get( 'default_manage_user_prefix' ) ) );
 $f_page_number   = gpc_get_int( 'page_number', 1 );
 
 if( !$f_save && !is_blank( gpc_get_cookie( $t_cookie_name, '' ) ) ) {