diff --git a/wp-includes/kses.php b/wp-includes/kses.php
index bc8db94995..179aaebd5b 100644
--- a/wp-includes/kses.php
+++ b/wp-includes/kses.php
@@ -1152,9 +1152,18 @@ function wp_kses_attr( $element, $attr, $allowed_html, $allowed_protocols ) {
  * @return bool Whether or not the attribute is allowed.
  */
 function wp_kses_attr_check( &$name, &$value, &$whole, $vless, $element, $allowed_html ) {
-	$allowed_attr = $allowed_html[ strtolower( $element ) ];
+	$name_low    = strtolower( $name );
+	$element_low = strtolower( $element );
+
+	if ( ! isset( $allowed_html[ $element_low ] ) ) {
+		$name  = '';
+		$value = '';
+		$whole = '';
+		return false;
+	}
+
+	$allowed_attr = $allowed_html[ $element_low ];
 
-	$name_low = strtolower( $name );
 	if ( ! isset( $allowed_attr[ $name_low ] ) || '' == $allowed_attr[ $name_low ] ) {
 		/*
 		 * Allow `data-*` attributes.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 73c0af47ab..15bab24d04 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.4-alpha-46958';
+$wp_version = '5.4-alpha-46959';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.