diff --git a/src/cdk/accordion/accordion.ts b/src/cdk/accordion/accordion.ts
index 3127c869a67a..3cbe7cc3e5a0 100644
--- a/src/cdk/accordion/accordion.ts
+++ b/src/cdk/accordion/accordion.ts
@@ -36,7 +36,7 @@ export class CdkAccordion implements OnDestroy, OnChanges {
   readonly _openCloseAllActions: Subject<boolean> = new Subject<boolean>();
 
   /** A readonly id value to use for unique selection coordination. */
-  readonly id = `cdk-accordion-${nextId++}`;
+  readonly id: string = `cdk-accordion-${nextId++}`;
 
   /** Whether the accordion should allow multiple expanded accordion items simultaneously. */
   @Input()