diff --git a/packages/@aws-cdk/aws-ec2/lib/instance.ts b/packages/@aws-cdk/aws-ec2/lib/instance.ts
index cbac936427ab7..1cd3d73a62c9f 100644
--- a/packages/@aws-cdk/aws-ec2/lib/instance.ts
+++ b/packages/@aws-cdk/aws-ec2/lib/instance.ts
@@ -291,7 +291,7 @@ export class Instance extends Resource implements IInstance {
       if (selected.length === 1) {
         subnet = selected[0];
       } else {
-        throw new Error('When specifying AZ there has to be exactly one subnet of the given type in this az');
+        throw new Error(`Need exactly 1 subnet to match AZ '${props.availabilityZone}', found ${selected.length}. Use a different availabilityZone.`);
       }
     } else {
       subnet = subnets[0];
diff --git a/packages/@aws-cdk/aws-ec2/lib/vpc.ts b/packages/@aws-cdk/aws-ec2/lib/vpc.ts
index a7fa786066abb..0ad96716bd562 100644
--- a/packages/@aws-cdk/aws-ec2/lib/vpc.ts
+++ b/packages/@aws-cdk/aws-ec2/lib/vpc.ts
@@ -1910,13 +1910,13 @@ const DUMMY_VPC_PROPS: cxapi.VpcContextResponse = {
       type: cxapi.VpcSubnetGroupType.PUBLIC,
       subnets: [
         {
-          availabilityZone: 'dummy-1a',
+          availabilityZone: 'dummy1a',
           subnetId: 's-12345',
           routeTableId: 'rtb-12345s',
           cidr: '1.2.3.4/5',
         },
         {
-          availabilityZone: 'dummy-1b',
+          availabilityZone: 'dummy1b',
           subnetId: 's-67890',
           routeTableId: 'rtb-67890s',
           cidr: '1.2.3.4/5',
@@ -1928,13 +1928,13 @@ const DUMMY_VPC_PROPS: cxapi.VpcContextResponse = {
       type: cxapi.VpcSubnetGroupType.PRIVATE,
       subnets: [
         {
-          availabilityZone: 'dummy-1a',
+          availabilityZone: 'dummy1a',
           subnetId: 'p-12345',
           routeTableId: 'rtb-12345p',
           cidr: '1.2.3.4/5',
         },
         {
-          availabilityZone: 'dummy-1b',
+          availabilityZone: 'dummy1b',
           subnetId: 'p-67890',
           routeTableId: 'rtb-57890p',
           cidr: '1.2.3.4/5',
diff --git a/packages/@aws-cdk/aws-ec2/test/test.vpc.from-lookup.ts b/packages/@aws-cdk/aws-ec2/test/test.vpc.from-lookup.ts
index 956f88f0aad0a..0c88693f1e7db 100644
--- a/packages/@aws-cdk/aws-ec2/test/test.vpc.from-lookup.ts
+++ b/packages/@aws-cdk/aws-ec2/test/test.vpc.from-lookup.ts
@@ -172,6 +172,22 @@ export = {
       restoreContextProvider(previous);
       test.done();
     },
+
+    'AZ in dummy lookup VPC matches AZ in Stack'(test: Test) {
+      // GIVEN
+      const stack = new Stack(undefined, 'MyTestStack', { env: { account: '1234567890', region: 'dummy' } });
+      const vpc = Vpc.fromLookup(stack, 'vpc', { isDefault: true });
+
+      // WHEN
+      const subnets = vpc.selectSubnets({
+        availabilityZones: stack.availabilityZones,
+      });
+
+      // THEN
+      test.equals(subnets.subnets.length, 2);
+
+      test.done();
+    },
   },
 };