diff --git a/src/Linkifier.test.ts b/src/Linkifier.test.ts
index 0e67403c..61245296 100644
--- a/src/Linkifier.test.ts
+++ b/src/Linkifier.test.ts
@@ -40,7 +40,7 @@ describe('Linkifier', () => {
     terminal = new MockTerminal();
     terminal.cols = 100;
     terminal.buffer = new MockBuffer();
-    terminal.buffer.lines = new CircularList<LineData>(20);
+    (<MockBuffer>terminal.buffer).setLines(new CircularList<LineData>(20));
     terminal.buffer.ydisp = 0;
     linkifier = new TestLinkifier(terminal);
     mouseZoneManager = new TestMouseZoneManager();
diff --git a/src/Types.ts b/src/Types.ts
index 84355e1f..2442b027 100644
--- a/src/Types.ts
+++ b/src/Types.ts
@@ -248,7 +248,7 @@ export interface ITerminalOptions extends IPublicTerminalOptions {
 }
 
 export interface IBuffer {
-  lines: ICircularList<LineData>;
+  readonly lines: ICircularList<LineData>;
   ydisp: number;
   ybase: number;
   y: number;
diff --git a/src/utils/TestUtils.test.ts b/src/utils/TestUtils.test.ts
index 4436c526..51aafb50 100644
--- a/src/utils/TestUtils.test.ts
+++ b/src/utils/TestUtils.test.ts
@@ -309,6 +309,9 @@ export class MockBuffer implements IBuffer {
   prevStop(x?: number): number {
     throw new Error('Method not implemented.');
   }
+  setLines(lines: ICircularList<[number, string, number, number][]>): void {
+    this.lines = lines;
+  }
 }
 
 export class MockRenderer implements IRenderer {