src/bytes/bytes_js_wasm_test.go       | 21 +++++++++++++++++++++
 src/internal/bytealg/indexbyte_wasm.s |  4 ++--

diff --git a/src/bytes/bytes_js_wasm_test.go b/src/bytes/bytes_js_wasm_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..ad9db343184f99faee7b86b5c6ca14425d2c130b
--- /dev/null
+++ b/src/bytes/bytes_js_wasm_test.go
@@ -0,0 +1,21 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js && wasm
+
+package bytes_test
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestIssue65571(t *testing.T) {
+	b := make([]byte, 1<<31+1)
+	b[1<<31] = 1
+	i := bytes.IndexByte(b, 1)
+	if i != 1<<31 {
+		t.Errorf("IndexByte(b, 1) = %d; want %d", i, 1<<31)
+	}
+}
diff --git a/src/internal/bytealg/indexbyte_wasm.s b/src/internal/bytealg/indexbyte_wasm.s
index ef4bd93070f44cc185b124369496dd8b33dd1ca4..d22e90448d43f6077f3b71f6407ad28ee9c599c7 100644
--- a/src/internal/bytealg/indexbyte_wasm.s
+++ b/src/internal/bytealg/indexbyte_wasm.s
@@ -12,7 +12,7 @@ 	I32Load8U c+24(FP)
 	I64Load b_len+8(FP)
 	I32WrapI64
 	Call memchr<>(SB)
-	I64ExtendI32S
+	I64ExtendI32U
 	Set R0
 
 	Get SP
@@ -35,7 +35,7 @@ 	I32Load8U c+16(FP)
 	I64Load s_len+8(FP)
 	I32WrapI64
 	Call memchr<>(SB)
-	I64ExtendI32S
+	I64ExtendI32U
 	Set R0
 
 	I64Const $-1