diff --git a/lib/object.js b/lib/object.js
index ff8e0dbd..9c4c3aff 100644
--- a/lib/object.js
+++ b/lib/object.js
@@ -127,14 +127,25 @@ class PDFObject {
 
       return `(${string})`;
     } else if (Array.isArray(object)) {
+      // Array entries are positional, so an `undefined` hole cannot simply be
+      // dropped without shifting everything after it. `null` is a real PDF
+      // object (ISO 32000-1, 7.3.9) and is the closest valid stand-in, which
+      // is also what JSON.stringify does.
       const items = object
-        .map((e) => PDFObject.convert(e, encryptFn))
+        .map((e) => PDFObject.convert(e === undefined ? null : e, encryptFn))
         .join(' ');
       return `[${items}]`;
     } else if ({}.toString.call(object) === '[object Object]') {
       const out = ['<<'];
       for (let key in object) {
         const val = object[key];
+        // `undefined` has no PDF representation, so serialising it produced the
+        // literal token `undefined` and an unparseable file. The spec treats a
+        // dictionary entry whose value is null as absent (ISO 32000-1, 7.3.9),
+        // so omitting the key is the closest valid equivalent - and it matches
+        // JSON.stringify. An explicit `null` is left alone: it is a real PDF
+        // object and callers may be relying on it.
+        if (val === undefined) continue;
         out.push(`/${key} ${PDFObject.convert(val, encryptFn)}`);
       }
 
diff --git a/tests/unit/object.spec.js b/tests/unit/object.spec.js
index 369c585b..c5396aa8 100644
--- a/tests/unit/object.spec.js
+++ b/tests/unit/object.spec.js
@@ -19,6 +19,20 @@ describe('PDFObject', () => {
       expect(result.length).toEqual(12);
       expect(result).toMatchInlineSnapshot(`"(þÿ±²³´)"`);
     });
+
+    test('dictionary omits keys whose value is undefined', () => {
+      expect(PDFObject.convert({ a: 1, b: undefined, c: 2 })).toEqual(
+        '<<\n/a 1\n/c 2\n>>',
+      );
+    });
+
+    test('dictionary keeps an explicit null', () => {
+      expect(PDFObject.convert({ a: null })).toEqual('<<\n/a null\n>>');
+    });
+
+    test('array converts an undefined entry to null to keep positions', () => {
+      expect(PDFObject.convert([1, undefined, 2])).toEqual('[1 null 2]');
+    });
   });
 
   describe('escapeName', () => {