Index: src/wp-includes/option.php
===================================================================
--- src/wp-includes/option.php	(revision 42819)
+++ src/wp-includes/option.php	(working copy)
@@ -1247,7 +1247,7 @@
 	$notoptions_key = "$network_id:notoptions";
 	$notoptions     = wp_cache_get( $notoptions_key, 'site-options' );
 
-	if ( isset( $notoptions[ $option ] ) ) {
+	if ( is_array( $notoptions ) && isset( $notoptions[ $option ] ) ) {
 
 		/**
 		 * Filters a specific default network option.
@@ -1295,6 +1295,11 @@
 		}
 	}
 
+	if ( ! is_array( $notoptions ) ) {
+		$notoptions = array();
+		wp_cache_set( $notoptions_key, $notoptions, 'site-options' );
+	}
+
 	/**
 	 * Filters the value of an existing network option.
 	 *
Index: tests/phpunit/tests/option/networkOption.php
===================================================================
--- tests/phpunit/tests/option/networkOption.php	(revision 42819)
+++ tests/phpunit/tests/option/networkOption.php	(working copy)
@@ -121,4 +121,50 @@
 			array( 'string', false ),
 		);
 	}
+
+	/**
+	 * @ticket 43506
+	 */
+	public function test_get_network_option_sets_notoptions_if_option_found() {
+		$network_id     = get_current_network_id();
+		$notoptions_key = "$network_id:notoptions";
+
+		$original_cache = wp_cache_get( $notoptions_key, 'site-options' );
+		if ( false !== $original_cache ) {
+			wp_cache_delete( $notoptions_key, 'site-options' );
+		}
+
+		// Retrieve any existing option.
+		get_network_option( $network_id, 'site_name' );
+
+		$cache = wp_cache_get( $notoptions_key, 'site-options' );
+		if ( false !== $original_cache ) {
+			wp_cache_set( $notoptions_key, $original_cache, 'site-options' );
+		}
+
+		$this->assertSame( array(), $cache );
+	}
+
+	/**
+	 * @ticket 43506
+	 */
+	public function test_get_network_option_sets_notoptions_if_option_not_found() {
+		$network_id     = get_current_network_id();
+		$notoptions_key = "$network_id:notoptions";
+
+		$original_cache = wp_cache_get( $notoptions_key, 'site-options' );
+		if ( false !== $original_cache ) {
+			wp_cache_delete( $notoptions_key, 'site-options' );
+		}
+
+		// Retrieve any non-existing option.
+		get_network_option( $network_id, 'this_does_not_exist' );
+
+		$cache = wp_cache_get( $notoptions_key, 'site-options' );
+		if ( false !== $original_cache ) {
+			wp_cache_set( $notoptions_key, $original_cache, 'site-options' );
+		}
+
+		$this->assertSame( array( 'this_does_not_exist' => true ), $cache );
+	}
 }