diff --git a/src/wp-admin/includes/ajax-actions.php b/src/wp-admin/includes/ajax-actions.php
index db22db2924..f11b243e1a 100644
--- a/src/wp-admin/includes/ajax-actions.php
+++ b/src/wp-admin/includes/ajax-actions.php
@@ -1443,9 +1443,6 @@ function wp_ajax_add_meta() {
 		if ( '' == trim( $key ) ) {
 			wp_die( __( 'Please provide a custom field name.' ) );
 		}
-		if ( '' == trim( $value ) ) {
-			wp_die( __( 'Please provide a custom field value.' ) );
-		}
 		if ( ! $meta = get_metadata_by_mid( 'post', $mid ) ) {
 			wp_die( 0 ); // if meta doesn't exist
 		}
diff --git a/src/wp-admin/includes/post.php b/src/wp-admin/includes/post.php
index 2595f69f7c..adc17ba91f 100644
--- a/src/wp-admin/includes/post.php
+++ b/src/wp-admin/includes/post.php
@@ -867,7 +867,7 @@ function add_meta( $post_ID ) {
 		$metavalue = trim( $metavalue );
 	}
 
-	if ( ( '0' === $metavalue || ! empty( $metavalue ) ) && ( ( ( '#NONE#' != $metakeyselect ) && ! empty( $metakeyselect ) ) || ! empty( $metakeyinput ) ) ) {
+	if ( ( ( '#NONE#' != $metakeyselect ) && ! empty( $metakeyselect ) ) || ! empty( $metakeyinput ) ) {
 		/*
 		 * We have a key/value pair. If both the select and the input
 		 * for the key have data, the input takes precedence.
diff --git a/tests/phpunit/tests/admin/includesPost.php b/tests/phpunit/tests/admin/includesPost.php
index 289f828e4c..ee3edb8aa0 100644
--- a/tests/phpunit/tests/admin/includesPost.php
+++ b/tests/phpunit/tests/admin/includesPost.php
@@ -799,4 +799,21 @@ class Tests_Admin_Includes_Post extends WP_UnitTestCase {
 		$this->assertArrayHasKey( $name, $blocks );
 		$this->assertSame( array( 'icon' => 'text' ), $blocks[ $name ] );
 	}
+
+	/**
+	 * @ticket 43559
+	 */
+	public function test_post_add_meta_empty_is_allowed() {
+		$p = self::factory()->post->create();
+
+		$_POST = array(
+			'metakeyinput' => 'testkey',
+			'metavalue'    => '',
+		);
+
+		wp_set_current_user( self::$admin_id );
+
+		$this->assertNotFalse( add_meta( $p ) );
+		$this->assertEquals( '', get_post_meta( $p, 'testkey', true ) );
+	}
 }
diff --git a/tests/phpunit/tests/ajax/AddMeta.php b/tests/phpunit/tests/ajax/AddMeta.php
new file mode 100644
index 0000000000..6ca4f3a0f2
--- /dev/null
+++ b/tests/phpunit/tests/ajax/AddMeta.php
@@ -0,0 +1,71 @@
+<?php
+
+/**
+ * Admin ajax functions to be tested
+ */
+require_once( ABSPATH . 'wp-admin/includes/ajax-actions.php' );
+
+/**
+ * Testing Add Meta AJAX functionality.
+ *
+ * @group ajax
+ */
+class Tests_Ajax_AddMeta extends WP_Ajax_UnitTestCase {
+	/**
+	 * @ticket 43559
+	 */
+	public function test_post_add_meta_empty_is_allowed_ajax() {
+		$p = self::factory()->post->create();
+
+		// Become an administrator.
+		$this->_setRole( 'administrator' );
+
+		$_POST = array(
+			'post_id'              => $p,
+			'metakeyinput'         => 'testkey',
+			'metavalue'            => '',
+			'_ajax_nonce-add-meta' => wp_create_nonce( 'add-meta' ),
+		);
+
+		// Make the request.
+		try {
+			$this->_handleAjax( 'add-meta' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			unset( $e );
+		}
+
+		$this->assertEquals( '', get_post_meta( $p, 'testkey', true ) );
+	}
+
+	/**
+	 * @ticket 43559
+	 */
+	public function test_post_update_meta_empty_is_allowed_ajax() {
+		$p = self::factory()->post->create();
+
+		$m = add_post_meta( $p, 'testkey', 'hello' );
+
+		// Become an administrator.
+		$this->_setRole( 'administrator' );
+
+		$_POST = array(
+			'_ajax_nonce-add-meta' => wp_create_nonce( 'add-meta' ),
+			'post_id'              => $p,
+			'meta'                 => array(
+				$m => array(
+					'key'   => 'testkey',
+					'value' => '',
+				),
+			),
+		);
+
+		// Make the request.
+		try {
+			$this->_handleAjax( 'add-meta' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			unset( $e );
+		}
+
+		$this->assertEquals( '', get_post_meta( $p, 'testkey', true ) );
+	}
+}