diff --git a/src/utils/mergeMap.ts b/src/utils/mergeMap.ts
index 21ad0086..b5d9b524 100644
--- a/src/utils/mergeMap.ts
+++ b/src/utils/mergeMap.ts
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-export function mergeMap<K, V>(src: Map<K, V> | undefined, dst: Map<K, V>) {
+export function mergeMap<K, V>(src: Map<K, V> | undefined, dst: Map<K, V>): void {
     if (src) {
         for (const [key, value] of src.entries()) {
             dst.set(key, value);