diff --git a/types/istanbul-lib-source-maps/index.d.ts b/types/istanbul-lib-source-maps/index.d.ts
index 45f7f3d05d..066beae58a 100644
--- a/types/istanbul-lib-source-maps/index.d.ts
+++ b/types/istanbul-lib-source-maps/index.d.ts
@@ -33,7 +33,7 @@ export interface MapStore {
     getSourceMapSync(filePath: string): any;
     addInputSourceMapsSync(coverageData: any): void;
     sourceFinder(filePath: string): string;
-    transformCoverage(coverageMap: CoverageMap): CoverageMap;
+    transformCoverage(coverageMap: CoverageMap): Promise<CoverageMap>;
     dispose(): void;
 }
 
diff --git a/types/istanbul-lib-source-maps/istanbul-lib-source-maps-tests.ts b/types/istanbul-lib-source-maps/istanbul-lib-source-maps-tests.ts
index 68fdf24fa8..310c823245 100644
--- a/types/istanbul-lib-source-maps/istanbul-lib-source-maps-tests.ts
+++ b/types/istanbul-lib-source-maps/istanbul-lib-source-maps-tests.ts
@@ -26,7 +26,7 @@ store.registerURL('foo', 'foo');
 
 const map = new CoverageMap({});
 const transformed = store.transformCoverage(map);
-transformed.data;
+transformed.then(obj => obj.data);
 
 store.dispose();