diff --git a/addons/addon-search/src/DecorationManager.ts b/addons/addon-search/src/DecorationManager.ts
index a8ec12017..3da46cc52 100644
--- a/addons/addon-search/src/DecorationManager.ts
+++ b/addons/addon-search/src/DecorationManager.ts
@@ -131,6 +131,9 @@ export class DecorationManager extends Disposable {
     const decorations: IDecoration[] = [];
     for (const range of decorationRanges) {
       const marker = this._terminal.registerMarker(range[0]);
+      if (!marker) {
+        continue;
+      }
       const decoration = this._terminal.registerDecoration({
         marker,
         x: range[1],
diff --git a/demo/client/components/window/testWindow.ts b/demo/client/components/window/testWindow.ts
index 1ea6b3e67..bb03bcf73 100644
--- a/demo/client/components/window/testWindow.ts
+++ b/demo/client/components/window/testWindow.ts
@@ -753,7 +753,7 @@ async function loadTestLongLines(term: Terminal, addons: AddonCollection): Promi
 
 function addDecoration(term: Terminal, dim: number = 1): void {
   term.options.scrollbar = { ...(term.options.scrollbar ?? {}), width: 14, overviewRuler: term.options.scrollbar?.overviewRuler ?? {} };
-  const marker = term.registerMarker(1);
+  const marker = term.registerMarker(1)!;
   const decoration = term.registerDecoration({
     marker,
     height: dim,
@@ -770,14 +770,14 @@ function addDecoration(term: Terminal, dim: number = 1): void {
 
 function addOverviewRuler(term: Terminal): void {
   term.options.scrollbar = { ...(term.options.scrollbar ?? {}), width: 14, overviewRuler: term.options.scrollbar?.overviewRuler ?? {} };
-  term.registerDecoration({ marker: term.registerMarker(1), overviewRulerOptions: { color: '#ef2929' } });
-  term.registerDecoration({ marker: term.registerMarker(3), overviewRulerOptions: { color: '#8ae234' } });
-  term.registerDecoration({ marker: term.registerMarker(5), overviewRulerOptions: { color: '#729fcf' } });
-  term.registerDecoration({ marker: term.registerMarker(7), overviewRulerOptions: { color: '#ef2929', position: 'left' } });
-  term.registerDecoration({ marker: term.registerMarker(7), overviewRulerOptions: { color: '#8ae234', position: 'center' } });
-  term.registerDecoration({ marker: term.registerMarker(7), overviewRulerOptions: { color: '#729fcf', position: 'right' } });
-  term.registerDecoration({ marker: term.registerMarker(10), overviewRulerOptions: { color: '#8ae234', position: 'center' } });
-  term.registerDecoration({ marker: term.registerMarker(10), overviewRulerOptions: { color: '#ffffff80', position: 'full' } });
+  term.registerDecoration({ marker: term.registerMarker(1)!, overviewRulerOptions: { color: '#ef2929' } });
+  term.registerDecoration({ marker: term.registerMarker(3)!, overviewRulerOptions: { color: '#8ae234' } });
+  term.registerDecoration({ marker: term.registerMarker(5)!, overviewRulerOptions: { color: '#729fcf' } });
+  term.registerDecoration({ marker: term.registerMarker(7)!, overviewRulerOptions: { color: '#ef2929', position: 'left' } });
+  term.registerDecoration({ marker: term.registerMarker(7)!, overviewRulerOptions: { color: '#8ae234', position: 'center' } });
+  term.registerDecoration({ marker: term.registerMarker(7)!, overviewRulerOptions: { color: '#729fcf', position: 'right' } });
+  term.registerDecoration({ marker: term.registerMarker(10)!, overviewRulerOptions: { color: '#8ae234', position: 'center' } });
+  term.registerDecoration({ marker: term.registerMarker(10)!, overviewRulerOptions: { color: '#ffffff80', position: 'full' } });
 }
 
 let decorationStressTestDecorations: IDisposable[] | undefined;
@@ -795,7 +795,7 @@ function decorationStressTest(term: Terminal): void {
       for (let y = 0; y < term.buffer.active.length; y++) {
         const cursorOffsetY = y - cursorY;
         const decoration = term.registerDecoration({
-          marker: term.registerMarker(cursorOffsetY),
+          marker: term.registerMarker(cursorOffsetY)!,
           x,
           width: 4,
           backgroundColor: '#FF0000',
diff --git a/src/browser/CoreBrowserTerminal.ts b/src/browser/CoreBrowserTerminal.ts
index e44b1717e..f22f5ac8e 100644
--- a/src/browser/CoreBrowserTerminal.ts
+++ b/src/browser/CoreBrowserTerminal.ts
@@ -768,7 +768,12 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
     return this.buffer.markers;
   }
 
-  public registerMarker(cursorYOffset: number): IMarker {
+  public registerMarker(cursorYOffset: number): IMarker | undefined {
+    // Disallow markers on the alt buffer
+    if (this.buffer !== this.buffers.normal) {
+      return;
+    }
+
     return this.buffer.addMarker(this.buffer.ybase + this.buffer.y + cursorYOffset);
   }
 
diff --git a/src/browser/Terminal.test.ts b/src/browser/Terminal.test.ts
index d359fa28c..bf9f616e2 100644
--- a/src/browser/Terminal.test.ts
+++ b/src/browser/Terminal.test.ts
@@ -1146,6 +1146,16 @@ describe('Terminal', () => {
     assert.equal(termConverting.buffer.lines.get(1)!.translateToString(true), 'World');
   });
 
+  describe('registerMarker', () => {
+    it('should return undefined on the alt buffer', async () => {
+      assert.isDefined(term.registerMarker(0));
+      await term.writeP('\x1b[?47h');
+      assert.isUndefined(term.registerMarker(0));
+      await term.writeP('\x1b[?47l');
+      assert.isDefined(term.registerMarker(0));
+    });
+  });
+
   // FIXME: move to common/CoreTerminal.test once the trimming is moved over
   describe('marker lifecycle', () => {
     // create a 10x5 terminal with markers on every line
diff --git a/src/browser/public/Terminal.ts b/src/browser/public/Terminal.ts
index 75aada048..b7e3c6c2c 100644
--- a/src/browser/public/Terminal.ts
+++ b/src/browser/public/Terminal.ts
@@ -166,7 +166,7 @@ export class Terminal extends Disposable implements ITerminalApi {
   public deregisterCharacterJoiner(joinerId: number): void {
     this._core.deregisterCharacterJoiner(joinerId);
   }
-  public registerMarker(cursorYOffset: number = 0): IMarker {
+  public registerMarker(cursorYOffset: number = 0): IMarker | undefined {
     this._verifyIntegers(cursorYOffset);
     return this._core.registerMarker(cursorYOffset);
   }
diff --git a/src/headless/public/Terminal.test.ts b/src/headless/public/Terminal.test.ts
index 0c4739744..21837d3a8 100644
--- a/src/headless/public/Terminal.test.ts
+++ b/src/headless/public/Terminal.test.ts
@@ -419,6 +419,18 @@ describe('Headless API Tests', function (): void {
       strictEqual(term.buffer.normal.getLine(0)!.translateToString(), 'norm ');
       strictEqual(term.buffer.alternate.getLine(0), undefined);
     });
+
+    it('registerMarker returns undefined on the alt buffer', async () => {
+      term = new Terminal({ cols: 5, allowProposedApi: true });
+      strictEqual(term.buffer.active.type, 'normal');
+      strictEqual(term.registerMarker(0)!.line, 0);
+      await writeSync('\x1b[?47h');
+      strictEqual(term.buffer.active.type, 'alternate');
+      strictEqual(term.registerMarker(0), undefined);
+      await writeSync('\x1b[?47l');
+      strictEqual(term.buffer.active.type, 'normal');
+      strictEqual(term.registerMarker(0)!.line, 0);
+    });
   });
 
   describe('modes', () => {
diff --git a/test/playwright/TestUtils.ts b/test/playwright/TestUtils.ts
index 8a76c8820..7dcdbe19b 100644
--- a/test/playwright/TestUtils.ts
+++ b/test/playwright/TestUtils.ts
@@ -284,7 +284,7 @@ export class TerminalProxy implements ITerminalProxyCustomMethods, PlaywrightApi
   }
   public async input(data: string, wasUserInput: boolean = true): Promise<void> { return this.evaluate(([term]) => term.input(data, wasUserInput)); }
   public async resize(cols: number, rows: number): Promise<void> { return this._page.evaluate(([term, cols, rows]) => term.resize(cols, rows), [await this.getHandle(), cols, rows] as const); }
-  public async registerMarker(y?: number | undefined): Promise<IMarker> { return this._page.evaluate(([term, y]) => term.registerMarker(y), [await this.getHandle(), y] as const); }
+  public async registerMarker(y?: number | undefined): Promise<IMarker | undefined> { return this._page.evaluate(([term, y]) => term.registerMarker(y), [await this.getHandle(), y] as const); }
   public async registerDecoration(decorationOptions: IDecorationOptions): Promise<IDecoration | undefined> { return this._page.evaluate(([term, decorationOptions]) => term.registerDecoration(decorationOptions), [await this.getHandle(), decorationOptions] as const); }
   public async clearTextureAtlas(): Promise<void> { return this.evaluate(([term]) => term.clearTextureAtlas()); }
   // #endregion
diff --git a/typings/xterm.d.ts b/typings/xterm.d.ts
index d1ce877b9..ab26dcd63 100644
--- a/typings/xterm.d.ts
+++ b/typings/xterm.d.ts
@@ -1257,11 +1257,12 @@ declare module '@xterm/xterm' {
     deregisterCharacterJoiner(joinerId: number): void;
 
     /**
-     * Adds a marker to the normal buffer and returns it.
+     * Adds a marker to the normal buffer and returns it. If the alt buffer is
+     * active, undefined is returned.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */
-    registerMarker(cursorYOffset?: number): IMarker;
+    registerMarker(cursorYOffset?: number): IMarker | undefined;
 
     /**
      * Registers a decoration to the terminal.