diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index f43f5e9a94..069e1c9ee7 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -1665,7 +1665,7 @@ function wp_insert_user( $userdata ) {
 
 	$user_activation_key = empty( $userdata['user_activation_key'] ) ? '' : $userdata['user_activation_key'];
 
-	if ( isset( $userdata['spam'] ) && ! is_multisite() ) {
+	if ( ! empty( $userdata['spam'] ) && ! is_multisite() ) {
 		return new WP_Error( 'no_spam', __( 'Sorry, marking a user as spam is only supported on Multisite.' ) );
 	}
 
diff --git a/tests/phpunit/tests/user.php b/tests/phpunit/tests/user.php
index faf6798ba7..d90793958b 100644
--- a/tests/phpunit/tests/user.php
+++ b/tests/phpunit/tests/user.php
@@ -574,6 +574,34 @@ class Tests_User extends WP_UnitTestCase {
 		$this->assertEquals( $pwd_before, $pwd_after );
 	}
 
+	/**
+	 * @ticket 45747
+	 */
+	function test_wp_update_user_should_not_mark_user_as_spam_on_single_site() {
+		if ( is_multisite() ) {
+			$this->markTestSkipped( 'This test is intended for single site.' );
+		}
+
+		$u = wp_update_user(
+			array(
+				'ID'   => self::$contrib_id,
+				'spam' => '0',
+			)
+		);
+
+		$this->assertNotWPError( $u );
+
+		$u = wp_update_user(
+			array(
+				'ID'   => self::$contrib_id,
+				'spam' => '1',
+			)
+		);
+
+		$this->assertWPError( $u );
+		$this->assertSame( 'no_spam', $u->get_error_code() );
+	}
+
 	/**
 	 * @ticket 28315
 	 */