Should be able to create an RDS cluster with a single instance

packages/@aws-cdk/aws-rds/lib/cluster.ts

@@ -177,7 +177,7 @@ export class DatabaseCluster extends DatabaseClusterRef {
         this.readerEndpoint = new Endpoint(cluster.dbClusterReadEndpointAddress, cluster.dbClusterEndpointPort);
 
         const instanceCount = props.instances != null ? props.instances : 2;
-        if (instanceCount <= 1) {
+        if (instanceCount < 1) {
             throw new Error('At least one instance is required');
         }
 

packages/@aws-cdk/aws-rds/test/test.cluster.ts

@@ -57,7 +57,37 @@ export = {
         // THEN: No error
 
         test.done();
-    }
+    },
+    'can create a cluster with a single instance'(test: Test) {
+        // GIVEN
+        const stack = testStack();
+        const vpc = new ec2.VpcNetwork(stack, 'VPC');
+
+        // WHEN
+        new DatabaseCluster(stack, 'Database', {
+            engine: DatabaseClusterEngine.Aurora,
+            instances: 1,
+            masterUser: {
+                username: new Username('admin'),
+                password: new Password('tooshort'),
+            },
+            instanceProps: {
+                instanceType: new ec2.InstanceTypePair(ec2.InstanceClass.Burstable2, ec2.InstanceSize.Small),
+                vpc
+            }
+        });
+
+        // THEN
+        expect(stack).to(haveResource('AWS::RDS::DBCluster', {
+            Engine: "aurora",
+            DBSubnetGroupName: { Ref: "DatabaseSubnets56F17B9A" },
+            MasterUsername: "admin",
+            MasterUserPassword: "tooshort",
+            VpcSecurityGroupIds: [ {"Fn::GetAtt": ["DatabaseSecurityGroup5C91FDCB", "GroupId"]}]
+        }));
+
+        test.done();
+    },
 };
 
 function testStack() {