diff --git a/io/util.ts b/io/util.ts
index 3266e50180..185732b366 100644
--- a/io/util.ts
+++ b/io/util.ts
@@ -4,6 +4,7 @@ import { Buffer, Reader } from "deno";
 // from `src`.
 // Returns the number of bytes copied.
 export function copyBytes(dst: Uint8Array, src: Uint8Array, off = 0): number {
+  off = Math.max(0, Math.min(off, dst.byteLength));
   const r = dst.byteLength - off;
   if (src.byteLength > r) {
     src = src.subarray(0, r);
diff --git a/io/util_test.ts b/io/util_test.ts
new file mode 100644
index 0000000000..7318d887cc
--- /dev/null
+++ b/io/util_test.ts
@@ -0,0 +1,36 @@
+import { test, assert } from "../testing/mod.ts";
+import { copyBytes } from "./util.ts";
+
+test(function testCopyBytes() {
+  let dst = new Uint8Array(4);
+
+  dst.fill(0);
+  let src = Uint8Array.of(1, 2);
+  let len = copyBytes(dst, src, 0);
+  assert(len === 2);
+  assert.equal(dst, Uint8Array.of(1, 2, 0, 0));
+
+  dst.fill(0);
+  src = Uint8Array.of(1, 2);
+  len = copyBytes(dst, src, 1);
+  assert(len === 2);
+  assert.equal(dst, Uint8Array.of(0, 1, 2, 0));
+
+  dst.fill(0);
+  src = Uint8Array.of(1, 2, 3, 4, 5);
+  len = copyBytes(dst, src);
+  assert(len === 4);
+  assert.equal(dst, Uint8Array.of(1, 2, 3, 4));
+
+  dst.fill(0);
+  src = Uint8Array.of(1, 2);
+  len = copyBytes(dst, src, 100);
+  assert(len === 0);
+  assert.equal(dst, Uint8Array.of(0, 0, 0, 0));
+
+  dst.fill(0);
+  src = Uint8Array.of(3, 4);
+  len = copyBytes(dst, src, -2);
+  assert(len === 2);
+  assert.equal(dst, Uint8Array.of(3, 4, 0, 0));
+});
diff --git a/test.ts b/test.ts
index 33a6f8d9b1..b9fcd74e08 100755
--- a/test.ts
+++ b/test.ts
@@ -14,6 +14,7 @@ import "fs/path/resolve_test.ts";
 import "fs/path/zero_length_strings_test.ts";
 import "io/bufio_test.ts";
 import "io/ioutil_test.ts";
+import "io/util_test.ts";
 import "http/http_test.ts";
 import "http/file_server_test.ts";
 import "log/test.ts";