diff --git a/modules/ppcp-paypal-subscriptions/src/PayPalSubscriptionsModule.php b/modules/ppcp-paypal-subscriptions/src/PayPalSubscriptionsModule.php
index d0aefecaa..79934074f 100644
--- a/modules/ppcp-paypal-subscriptions/src/PayPalSubscriptionsModule.php
+++ b/modules/ppcp-paypal-subscriptions/src/PayPalSubscriptionsModule.php
@@ -80,6 +80,8 @@ class PayPalSubscriptionsModule implements ModuleInterface {
 					return;
 				}
 
+				$foo = $product;
+
 				$subscriptions_api_handler = $c->get( 'paypal-subscriptions.api-handler' );
 				assert( $subscriptions_api_handler instanceof SubscriptionsApiHandler );
 				$this->update_subscription_product_meta( $product, $subscriptions_api_handler );
@@ -87,6 +89,30 @@ class PayPalSubscriptionsModule implements ModuleInterface {
 			12
 		);
 
+		add_filter( 'woocommerce_add_to_cart_validation', function
+		(
+			$passed_validation,
+			$product_id
+		) {
+			if ( WC()->cart->is_empty() ) {
+				return $passed_validation;
+			}
+
+			foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
+				if ( get_post_meta( $cart_item['product_id'], 'ppcp_subscription_product', true ) ) {
+					wc_add_notice(__('You can only have one subscription product in your cart.', 'woocommerce-paypal-payments'), 'error');
+					return false;
+				}
+
+				if ( get_post_meta( $product_id, 'ppcp_subscription_product', true ) ) {
+					wc_add_notice(__('You cannot add a subscription product to a cart with other items.', 'woocommerce-paypal-payments'), 'error');
+					return false;
+				}
+			}
+
+			return $passed_validation;
+		}, 10, 2);
+
 		add_action(
 			'woocommerce_save_product_variation',
 			/**