$preText = $this->msg( 'tags-delete-explanation-initial', $tag )->parseAsBlock();
$tagUsage = ChangeTags::tagUsageStatistics();
- if ( $tagUsage[$tag] > 0 ) {
+ if ( isset( $tagUsage[$tag] ) && $tagUsage[$tag] > 0 ) {
$preText .= $this->msg( 'tags-delete-explanation-in-use', $tag,
$tagUsage[$tag] )->parseAsBlock();
}