diff --git a/README.md b/README.md
index 7624a37..c19e31e 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,10 @@ TypeScript2Python supports many of TypeScripts type constructs, including:
 
 ## Transpiler options
 
+### Strict
+
+Use the `--strict` flag to enable all strict type-checking options to ensure `undefined` and `null` properties are not ignored during transpilation.
+
 ### Nullable optionals
 
 In TypeScript objects, optional values can also be set to `undefined`. By default we assume the according Python
diff --git a/src/index.ts b/src/index.ts
index 2a76405..8477e36 100755
--- a/src/index.ts
+++ b/src/index.ts
@@ -5,13 +5,15 @@ import path from "path";
 import { program } from "@commander-js/extra-typings";
 import { typeScriptToPython } from "./typeScriptToPython";
 import { Ts2PyConfig } from "./config";
+import { readFileSync } from "fs";
 
-const compile = (fileNames: string[], config: Ts2PyConfig) => {
+const compile = (fileNames: string[], config: Ts2PyConfig & {strict?: boolean}) => {
   const program = ts.createProgram(fileNames, {
     noEmit: true,
     allowJs: true,
     resolveJsonModule: true,
     skipLibCheck: true,
+    strict: config.strict,
   });
 
   const relevantSourceFiles = program
@@ -21,7 +23,6 @@ const compile = (fileNames: string[], config: Ts2PyConfig) => {
         .map((fn) => path.relative(fn, f.fileName) === "")
         .reduce((a, b) => a || b),
     );
-
   const transpiled = typeScriptToPython(program.getTypeChecker(), relevantSourceFiles, config)
   console.log(transpiled);
 }
@@ -30,6 +31,7 @@ program
   .name("typescript2python")
   .description("A program that converts TypeScript type definitions to Python")
   .option("--nullable-optionals", "if set, optional entries in dictionaries will be nullable, e.g. `NotRequired[Optional[T]]`")
+  .option("--strict", "Enable all strict type-checking options.")
   .arguments("<input...>")
   .action((args, options) => {
     compile(args, options)
diff --git a/src/testing/basic.test.ts b/src/testing/basic.test.ts
index eb7ebdc..c0cd21b 100644
--- a/src/testing/basic.test.ts
+++ b/src/testing/basic.test.ts
@@ -57,11 +57,30 @@ describe("transpiling basic types", () => {
       "export type T = number | string | Record<string, boolean>",
       "from typing_extensions import Dict, Union\n\nT = Union[str,float,Dict[str,bool]]",
     ],
+    [
+      "export type T = number | undefined",
+      // without strict mode the `undefined` gets lost here
+      "T = float",
+    ],
   ])("transpiles %p to %p", async (input, expected) => {
     const result = await transpileString(input);
     expect(result).toEqual(expected);
   });
 
+  it.each([
+    [
+      "export type T = number | undefined",
+      "from typing_extensions import Union\n\nT = Union[None,float]",
+    ],
+    [
+      "export type T = number | null",
+      "from typing_extensions import Union\n\nT = Union[None,float]",
+    ],
+  ])("transpiles %p to %p when strict", async (input, expected) => {
+    const result = await transpileString(input, {}, { strict: true });
+    expect(result).toEqual(expected);
+  });
+
   it("only transpiles exported types", async () => {
     const result = await transpileString(`
       type NotExported = number; 
diff --git a/src/testing/dicts.test.ts b/src/testing/dicts.test.ts
index 73279b3..7438a1a 100644
--- a/src/testing/dicts.test.ts
+++ b/src/testing/dicts.test.ts
@@ -64,7 +64,7 @@ class A(TypedDict):
     expect(result).toContain(`class A(TypedDict):\n  foo: NotRequired[str]`);
   });
 
-  it("transpiles optional values with non-null optionals as NotRequired[T]", async () => {
+  it.only("transpiles optional values with non-null optionals as NotRequired[T]", async () => {
     const result = await transpileString(`export type A = { foo?: string }`, {
       nullableOptionals: true,
     });
diff --git a/src/testing/utils.ts b/src/testing/utils.ts
index 2eb5503..0b136e8 100644
--- a/src/testing/utils.ts
+++ b/src/testing/utils.ts
@@ -8,9 +8,13 @@ import { createProject, ts } from "@ts-morph/bootstrap";
  **/
 let globalProject: ReturnType<typeof createProject> | undefined;
 
+/** Each file should get a unique name to avoid issues. */
+let i = 0;
+
 export const transpileString = async (
   code: string,
   config: Ts2PyConfig = {},
+  compilerOptions: ts.CompilerOptions = {},
 ) => {
   if (globalProject === undefined) {
     globalProject = createProject({
@@ -19,11 +23,14 @@ export const transpileString = async (
   }
 
   const project = await globalProject;
-  const fileName = `source.ts`;
+  const fileName = `source_${i++}.ts`;
 
   // instead of adding a new source file for each program, we update the existing one.
   const sourceFile = project.updateSourceFile(fileName, code);
-  const program = project.createProgram();
+  const program = project.createProgram({
+    rootNames: [fileName],
+    options: { ...project.compilerOptions, ...compilerOptions },
+  });
   const diagnostics = ts.getPreEmitDiagnostics(program);
 
   if (diagnostics.length > 0) {