diff --git a/packages/@aws-cdk/aws-rds/lib/cluster.ts b/packages/@aws-cdk/aws-rds/lib/cluster.ts
index a3f06555afdb9..10a9e09e30045 100644
--- a/packages/@aws-cdk/aws-rds/lib/cluster.ts
+++ b/packages/@aws-cdk/aws-rds/lib/cluster.ts
@@ -651,6 +651,9 @@ interface InstanceConfig {
  */
 function createInstances(cluster: DatabaseClusterNew, props: DatabaseClusterBaseProps, subnetGroup: ISubnetGroup): InstanceConfig {
   const instanceCount = props.instances != null ? props.instances : 2;
+  if (Token.isUnresolved(instanceCount)) {
+    throw new Error('Instance count is not a token');
+  }
   if (instanceCount < 1) {
     throw new Error('At least one instance is required');
   }