Index: src/wp-includes/kses.php
===================================================================
--- src/wp-includes/kses.php	(revision 40570)
+++ src/wp-includes/kses.php	(working copy)
@@ -828,8 +828,9 @@
 		$xhtml_slash = ' /';
 
 	// Are any attributes allowed at all for this element?
-	if ( ! isset($allowed_html[strtolower($element)]) || count($allowed_html[strtolower($element)]) == 0 )
+	if ( ! isset( $allowed_html[ strtolower( $element ) ] ) || true === $allowed_html[ strtolower( $element ) ] || count( $allowed_html[ strtolower( $element ) ] ) == 0 ) {
 		return "<$element$xhtml_slash>";
+	}
 
 	// Split it
 	$attrarr = wp_kses_hair($attr, $allowed_protocols);
Index: tests/phpunit/tests/kses.php
===================================================================
--- tests/phpunit/tests/kses.php	(revision 40570)
+++ tests/phpunit/tests/kses.php	(working copy)
@@ -678,4 +678,34 @@
 
 		$this->assertEquals( $input, wp_kses( $input, $allowedposttags ) );
 	}
+
+	/**
+	 * @ticket 40680
+	 */
+	function test_wp_kses_attr_no_attributes_allowed_with_empty_array() {
+		$element = 'foo';
+		$attribute = 'title="foo" class="bar"';
+
+		$this->assertEquals( "<{$element}>", wp_kses_attr( $element, $attribute, array( 'foo' => array() ), array() ) );
+	}
+
+	/**
+	 * @ticket 40680
+	 */
+	function test_wp_kses_attr_no_attributes_allowed_with_true() {
+		$element = 'foo';
+		$attribute = 'title="foo" class="bar"';
+
+		$this->assertEquals( "<{$element}>", wp_kses_attr( $element, $attribute, array( 'foo' => true ), array() ) );
+	}
+
+	/**
+	 * @ticket 40680
+	 */
+	function test_wp_kses_attr_single_attribute_is_allowed() {
+		$element = 'foo';
+		$attribute = 'title="foo" class="bar"';
+
+		$this->assertEquals( "<{$element} title=\"foo\">", wp_kses_attr( $element, $attribute, array( 'foo' => array( 'title' => true ) ), array() ) );
+	}
 }