diff --git a/src/headless/Terminal.ts b/src/headless/Terminal.ts
index e53c00fbb..a348ae824 100644
--- a/src/headless/Terminal.ts
+++ b/src/headless/Terminal.ts
@@ -69,12 +69,7 @@ export class Terminal extends CoreTerminal {
     return this.buffer.markers;
   }
 
-  public registerMarker(cursorYOffset: number): IMarker | undefined {
-    // Disallow markers on the alt buffer
-    if (this.buffer !== this.buffers.normal) {
-      return;
-    }
-
+  public registerMarker(cursorYOffset: number): IMarker {
     return this.buffer.addMarker(this.buffer.ybase + this.buffer.y + cursorYOffset);
   }
 
diff --git a/src/headless/public/Terminal.test.ts b/src/headless/public/Terminal.test.ts
index 0c4739744..73e2f4865 100644
--- a/src/headless/public/Terminal.test.ts
+++ b/src/headless/public/Terminal.test.ts
@@ -120,10 +120,10 @@ describe('Headless API Tests', function (): void {
       await writeSync('\n\rtest' + i);
     }
     const markers = [
-      term.registerMarker(1)!,
-      term.registerMarker(2)!,
-      term.registerMarker(3)!,
-      term.registerMarker(4)!
+      term.registerMarker(1),
+      term.registerMarker(2),
+      term.registerMarker(3),
+      term.registerMarker(4)
     ];
     let disposeCount = 0;
     for (const marker of markers) {
@@ -419,6 +419,15 @@ describe('Headless API Tests', function (): void {
       strictEqual(term.buffer.normal.getLine(0)!.translateToString(), 'norm ');
       strictEqual(term.buffer.alternate.getLine(0), undefined);
     });
+
+    it('registerMarker on alternate buffer', async () => {
+      term = new Terminal({ cols: 5, allowProposedApi: true });
+      await writeSync('\x1b[?47h');
+      const marker = term.registerMarker(0);
+      strictEqual(term.buffer.active.type, 'alternate');
+      strictEqual(term.markers.length, 1);
+      strictEqual(term.markers[0], marker);
+    });
   });
 
   describe('modes', () => {
diff --git a/src/headless/public/Terminal.ts b/src/headless/public/Terminal.ts
index 87eaf3976..d593dea15 100644
--- a/src/headless/public/Terminal.ts
+++ b/src/headless/public/Terminal.ts
@@ -130,11 +130,11 @@ export class Terminal extends Disposable implements ITerminalApi {
     this._verifyIntegers(columns, rows);
     this._core.resize(columns, rows);
   }
-  public registerMarker(cursorYOffset: number = 0): IMarker | undefined {
+  public registerMarker(cursorYOffset: number = 0): IMarker {
     this._verifyIntegers(cursorYOffset);
     return this._core.registerMarker(cursorYOffset);
   }
-  public addMarker(cursorYOffset: number): IMarker | undefined {
+  public addMarker(cursorYOffset: number): IMarker {
     return this.registerMarker(cursorYOffset);
   }
   public dispose(): void {
diff --git a/typings/xterm-headless.d.ts b/typings/xterm-headless.d.ts
index d3b62346d..d51dc096d 100644
--- a/typings/xterm-headless.d.ts
+++ b/typings/xterm-headless.d.ts
@@ -825,12 +825,11 @@ declare module '@xterm/headless' {
     resize(columns: number, rows: number): void;
 
     /**
-     * Adds a marker to the normal buffer and returns it. If the alt buffer is
-     * active, undefined is returned.
+     * Adds a marker to the normal buffer and returns it.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */
-    registerMarker(cursorYOffset?: number): IMarker | undefined;
+    registerMarker(cursorYOffset?: number): IMarker;
 
     /*
      * Disposes of the terminal, detaching it from the DOM and removing any