diff --git src/wp-includes/option.php src/wp-includes/option.php
index a80de70..88a0a1f 100644
--- src/wp-includes/option.php
+++ src/wp-includes/option.php
@@ -1243,27 +1243,29 @@ function get_network_option( $network_id, $option, $default = false ) {
 		return $pre;
 	}
 
-	// prevent non-existent options from triggering multiple queries
-	$notoptions_key = "$network_id:notoptions";
-	$notoptions     = wp_cache_get( $notoptions_key, 'site-options' );
+	if ( is_multisite() ) {
+		// prevent non-existent options from triggering multiple queries
+		$notoptions_key = "$network_id:notoptions";
+		$notoptions     = wp_cache_get( $notoptions_key, 'site-options' );
 
-	if ( is_array( $notoptions ) && isset( $notoptions[ $option ] ) ) {
+		if ( is_array( $notoptions ) && isset( $notoptions[ $option ] ) ) {
 
-		/**
-		 * Filters a specific default network option.
-		 *
-		 * The dynamic portion of the hook name, `$option`, refers to the option name.
-		 *
-		 * @since 3.4.0
-		 * @since 4.4.0 The `$option` parameter was added.
-		 * @since 4.7.0 The `$network_id` parameter was added.
-		 *
-		 * @param mixed  $default    The value to return if the site option does not exist
-		 *                           in the database.
-		 * @param string $option     Option name.
-		 * @param int    $network_id ID of the network.
-		 */
-		return apply_filters( "default_site_option_{$option}", $default, $option, $network_id );
+			/**
+			 * Filters a specific default network option.
+			 *
+			 * The dynamic portion of the hook name, `$option`, refers to the option name.
+			 *
+			 * @since 3.4.0
+			 * @since 4.4.0 The `$option` parameter was added.
+			 * @since 4.7.0 The `$network_id` parameter was added.
+			 *
+			 * @param mixed  $default    The value to return if the site option does not exist
+			 *                           in the database.
+			 * @param string $option     Option name.
+			 * @param int    $network_id ID of the network.
+			 */
+			return apply_filters( "default_site_option_{$option}", $default, $option, $network_id );
+		}
 	}
 
 	if ( ! is_multisite() ) {
@@ -1295,7 +1297,7 @@ function get_network_option( $network_id, $option, $default = false ) {
 		}
 	}
 
-	if ( ! is_array( $notoptions ) ) {
+	if ( isset( $notoptions ) && ! is_array( $notoptions ) ) {
 		$notoptions = array();
 		wp_cache_set( $notoptions_key, $notoptions, 'site-options' );
 	}