diff --git a/wait_group.ts b/wait_group.ts
index b98d165..2f9bb46 100644
--- a/wait_group.ts
+++ b/wait_group.ts
@@ -36,6 +36,9 @@ export class WaitGroup {
    * @param delta The number to add to the counter. It can be positive or negative.
    */
   add(delta: number): void {
+    if (!Number.isSafeInteger(delta)) {
+      throw new RangeError(`delta must be a safe integer, got ${delta}`);
+    }
     this.#count += delta;
     if (this.#count === 0) {
       this.#notify.notifyAll();
diff --git a/wait_group_test.ts b/wait_group_test.ts
index 786cb20..bcbbca2 100644
--- a/wait_group_test.ts
+++ b/wait_group_test.ts
@@ -1,4 +1,4 @@
-import { assertEquals, assertRejects } from "@std/assert";
+import { assertEquals, assertRejects, assertThrows } from "@std/assert";
 import { deadline, delay } from "@std/async";
 import { WaitGroup } from "./wait_group.ts";
 
@@ -83,4 +83,15 @@ Deno.test("WaitGroup", async (t) => {
       );
     },
   );
+
+  await t.step(
+    "'add' throws RangeError if delta is not a safe integer",
+    () => {
+      const wg = new WaitGroup();
+      assertThrows(() => wg.add(NaN), RangeError);
+      assertThrows(() => wg.add(Infinity), RangeError);
+      assertThrows(() => wg.add(-Infinity), RangeError);
+      assertThrows(() => wg.add(1.1), RangeError);
+    },
+  );
 });