diff --git a/packages/@aws-cdk/aws-sam/test/sam.test.ts b/packages/@aws-cdk/aws-sam/test/application.test.ts
similarity index 100%
rename from packages/@aws-cdk/aws-sam/test/sam.test.ts
rename to packages/@aws-cdk/aws-sam/test/application.test.ts
diff --git a/packages/@aws-cdk/aws-sam/test/function.test.ts b/packages/@aws-cdk/aws-sam/test/function.test.ts
new file mode 100644
index 0000000000000..d8dade625be0e
--- /dev/null
+++ b/packages/@aws-cdk/aws-sam/test/function.test.ts
@@ -0,0 +1,27 @@
+import '@aws-cdk/assert/jest';
+import * as cdk from '@aws-cdk/core';
+import * as sam from '../lib';
+
+test("correctly chooses a string array from the type unions of the 'policies' property", () => {
+  const stack = new cdk.Stack();
+
+  new sam.CfnFunction(stack, 'MyFunction', {
+    codeUri: {
+      bucket: 'my-bucket',
+      key: 'my-key',
+    },
+    runtime: 'nodejs-12.x',
+    handler: 'index.handler',
+    policies: ['AWSLambdaExecute'],
+  });
+
+  expect(stack).toHaveResourceLike('AWS::Serverless::Function', {
+    CodeUri: {
+      Bucket: 'my-bucket',
+      Key: 'my-key',
+    },
+    Handler: 'index.handler',
+    Runtime: 'nodejs-12.x',
+    Policies: ['AWSLambdaExecute'],
+  });
+});
diff --git a/tools/cfn2ts/lib/codegen.ts b/tools/cfn2ts/lib/codegen.ts
index 8d85d05f7ee72..08d0636a841f5 100644
--- a/tools/cfn2ts/lib/codegen.ts
+++ b/tools/cfn2ts/lib/codegen.ts
@@ -684,6 +684,15 @@ export default class CodeGenerator {
 
     this.code.line(`const errors = new ${CORE}.ValidationResults();`);
 
+    // check that the argument is an object
+    // normally, we would have to explicitly check for null here,
+    // as typeof null is 'object' in JavaScript,
+    // but validators are never called with null
+    // (as evidenced by the code below accessing properties of the argument without checking for null)
+    this.code.openBlock("if (typeof properties !== 'object')");
+    this.code.line(`errors.collect(new ${CORE}.ValidationResult('Expected an object, but received: ' + JSON.stringify(properties)));`);
+    this.code.closeBlock();
+
     Object.keys(propSpecs).forEach(cfnPropName => {
       const propSpec = propSpecs[cfnPropName];
       const propName = nameConversionTable[cfnPropName];