diff --git a/ts/test-both/util/iterables_test.ts b/ts/test-both/util/iterables_test.ts
index a1fb9101e..82a77c4ef 100644
--- a/ts/test-both/util/iterables_test.ts
+++ b/ts/test-both/util/iterables_test.ts
@@ -4,7 +4,14 @@
 import { assert } from 'chai';
 import * as sinon from 'sinon';
 
-import { isIterable, size, filter, map, take } from '../../util/iterables';
+import {
+  concat,
+  filter,
+  isIterable,
+  map,
+  size,
+  take,
+} from '../../util/iterables';
 
 describe('iterable utilities', () => {
   describe('isIterable', () => {
@@ -82,6 +89,72 @@ describe('iterable utilities', () => {
     });
   });
 
+  describe('concat', () => {
+    it('returns an empty iterable when passed nothing', () => {
+      assert.deepEqual([...concat()], []);
+    });
+
+    it('returns an empty iterable when passed empty iterables', () => {
+      assert.deepEqual([...concat([])], []);
+      assert.deepEqual([...concat(new Set())], []);
+      assert.deepEqual([...concat(new Set(), [], new Map())], []);
+    });
+
+    it('concatenates multiple iterables', () => {
+      const everyNumber = {
+        *[Symbol.iterator]() {
+          for (let i = 4; true; i += 1) {
+            yield i;
+          }
+        },
+      };
+
+      const result = concat([1, 2], new Set([3]), [], everyNumber);
+      const iterator = result[Symbol.iterator]();
+
+      assert.deepEqual(iterator.next(), { value: 1, done: false });
+      assert.deepEqual(iterator.next(), { value: 2, done: false });
+      assert.deepEqual(iterator.next(), { value: 3, done: false });
+      assert.deepEqual(iterator.next(), { value: 4, done: false });
+      assert.deepEqual(iterator.next(), { value: 5, done: false });
+      assert.deepEqual(iterator.next(), { value: 6, done: false });
+      assert.deepEqual(iterator.next(), { value: 7, done: false });
+    });
+
+    it("doesn't start the iterable until the last minute", () => {
+      const oneTwoThree = {
+        [Symbol.iterator]: sinon.fake(() => {
+          let n = 0;
+          return {
+            next() {
+              if (n > 3) {
+                return { done: true };
+              }
+              n += 1;
+              return { value: n, done: false };
+            },
+          };
+        }),
+      };
+
+      const result = concat([1, 2], oneTwoThree);
+      const iterator = result[Symbol.iterator]();
+
+      sinon.assert.notCalled(oneTwoThree[Symbol.iterator]);
+
+      iterator.next();
+      sinon.assert.notCalled(oneTwoThree[Symbol.iterator]);
+      iterator.next();
+      sinon.assert.notCalled(oneTwoThree[Symbol.iterator]);
+
+      iterator.next();
+      sinon.assert.calledOnce(oneTwoThree[Symbol.iterator]);
+
+      iterator.next();
+      sinon.assert.calledOnce(oneTwoThree[Symbol.iterator]);
+    });
+  });
+
   describe('filter', () => {
     it('returns an empty iterable when passed an empty iterable', () => {
       const fn = sinon.fake();
diff --git a/ts/textsecure/SendMessage.ts b/ts/textsecure/SendMessage.ts
index e68436f20..df53d70d6 100644
--- a/ts/textsecure/SendMessage.ts
+++ b/ts/textsecure/SendMessage.ts
@@ -48,6 +48,7 @@ import {
   LinkPreviewImage,
   LinkPreviewMetadata,
 } from '../linkPreviews/linkPreviewFetch';
+import { concat } from '../util/iterables';
 
 function stringToArrayBuffer(str: string): ArrayBuffer {
   if (typeof str !== 'string') {
@@ -1706,10 +1707,12 @@ export default class MessageSender {
       isNotMe = r => r !== myE164;
     }
 
-    const blockedIdentifiers = new Set([
-      ...window.storage.getBlockedUuids(),
-      ...window.storage.getBlockedNumbers(),
-    ]);
+    const blockedIdentifiers = new Set(
+      concat(
+        window.storage.getBlockedUuids(),
+        window.storage.getBlockedNumbers()
+      )
+    );
 
     const recipients = groupMembers.filter(
       recipient => isNotMe(recipient) && !blockedIdentifiers.has(recipient)
diff --git a/ts/util/iterables.ts b/ts/util/iterables.ts
index 1664d592d..68d4df294 100644
--- a/ts/util/iterables.ts
+++ b/ts/util/iterables.ts
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 
 /* eslint-disable max-classes-per-file */
+/* eslint-disable no-restricted-syntax */
 
 export function isIterable(value: unknown): value is Iterable<unknown> {
   return (
@@ -28,6 +29,22 @@ export function size(iterable: Iterable<unknown>): number {
   return result;
 }
 
+export function concat<T>(
+  ...iterables: ReadonlyArray<Iterable<T>>
+): Iterable<T> {
+  return new ConcatIterable(iterables);
+}
+
+class ConcatIterable<T> implements Iterable<T> {
+  constructor(private readonly iterables: ReadonlyArray<Iterable<T>>) {}
+
+  *[Symbol.iterator](): Iterator<T> {
+    for (const iterable of this.iterables) {
+      yield* iterable;
+    }
+  }
+}
+
 export function filter<T, S extends T>(
   iterable: Iterable<T>,
   predicate: (value: T) => value is S