diff --git a/index.d.ts b/index.d.ts
index 7d2ec98..380e404 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -123,5 +123,5 @@ export class Transformer {
    * # Errors
    * Returns an error if the transformation fails to find injection points.
    */
-  transform(code: string | Buffer, moduleType: ModuleType, sourcemap?: string | null): TransformOutput;
+  transform(code: string | Buffer, moduleType: ModuleType, sourcemap?: string | object | null): TransformOutput;
 }
diff --git a/lib/transformer.js b/lib/transformer.js
index 663e3cb..76d6c4d 100644
--- a/lib/transformer.js
+++ b/lib/transformer.js
@@ -138,10 +138,17 @@ class Transformer {
       SourceMapGenerator ??= require('source-map').SourceMapGenerator
 
       const file = `${this.#moduleName}/${this.#filePath}`
-      const sourceMapInput = sourcemap ? new SourceMapConsumer(sourcemap) : { file }
-      const sourceMap = new SourceMapGenerator(sourceMapInput)
-      const code = generate(ast, { sourceMap })
-      const map = sourceMap.toString()
+      let generator
+      if (sourcemap) {
+        const consumer = new SourceMapConsumer(sourcemap)
+        consumer.file = file
+        generator = SourceMapGenerator.fromSourceMap(consumer)
+      } else {
+        generator = new SourceMapGenerator({ file })
+      }
+
+      const code = generate(ast, { sourceMap: generator })
+      const map = generator.toString()
 
       return { code, map }
     }
diff --git a/tests/tests.test.mjs b/tests/tests.test.mjs
index 225bcb8..e2eccdc 100644
--- a/tests/tests.test.mjs
+++ b/tests/tests.test.mjs
@@ -516,6 +516,23 @@ describe('source_map', () => {
     assert.equal(original.line, originalReturnLine)
     assert.equal(original.column, originalReturnColumn)
   })
+
+  test('produces a valid sourcemap when an inputSourceMap is provided', () => {
+    const code = 'export class Undici { async fetch (url) { return 42; } }'
+    const inputSourceMap = { version: 3, sources: ['input.js'], mappings: 'AAAA', names: [] }
+
+    const instrumentor = create([
+      {
+        channelName: 'Undici:fetch',
+        module: { name: TEST_MODULE_NAME, versionRange: '>=0.0.1', filePath: TEST_MODULE_PATH },
+        functionQuery: { className: 'Undici', methodName: 'fetch', kind: 'Async' },
+      },
+    ])
+    const transformer = instrumentor.getTransformer(TEST_MODULE_NAME, TEST_MODULE_VERSION, TEST_MODULE_PATH)
+    const { map } = transformer.transform(code, 'esm', inputSourceMap)
+
+    assert.equal(JSON.parse(map).file, `${TEST_MODULE_NAME}/${TEST_MODULE_PATH}`)
+  })
 })
 
 describe('wrap_promise_non_promise', () => {