diff --git a/Tests/LibWeb/Text/expected/Streams/ReadableStream-autoAllocateChunkSize-constructor.txt b/Tests/LibWeb/Text/expected/Streams/ReadableStream-autoAllocateChunkSize-constructor.txt
new file mode 100644
index 0000000000..1c2dbd9464
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/Streams/ReadableStream-autoAllocateChunkSize-constructor.txt
@@ -0,0 +1 @@
+PASS. Made: ReadableStream
diff --git a/Tests/LibWeb/Text/input/Streams/ReadableStream-autoAllocateChunkSize-constructor.html b/Tests/LibWeb/Text/input/Streams/ReadableStream-autoAllocateChunkSize-constructor.html
new file mode 100644
index 0000000000..73843944f9
--- /dev/null
+++ b/Tests/LibWeb/Text/input/Streams/ReadableStream-autoAllocateChunkSize-constructor.html
@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let stream = new ReadableStream({
+            type: "bytes",
+            autoAllocateChunkSize: 64
+        });
+        println(`PASS. Made: ${stream.constructor.name}`);
+    });
+</script>
diff --git a/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp
index ec6846f4ca..644f468b8d 100644
--- a/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp
+++ b/Userland/Libraries/LibWeb/Streams/UnderlyingSource.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
+ * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,7 +8,9 @@
 #include <LibJS/Runtime/VM.h>
 #include <LibWeb/Streams/AbstractOperations.h>
 #include <LibWeb/Streams/UnderlyingSource.h>
+#include <LibWeb/WebIDL/AbstractOperations.h>
 #include <LibWeb/WebIDL/CallbackType.h>
+#include <LibWeb/WebIDL/Types.h>
 
 namespace Web::Streams {
 
@@ -35,8 +38,10 @@ JS::ThrowCompletionOr<UnderlyingSource> UnderlyingSource::from_value(JS::VM& vm,
             return vm.throw_completion<JS::TypeError>(ByteString::formatted("Unknown stream type '{}'", type_value));
     }
 
-    if (TRY(object.has_property("autoAllocateChunkSize")))
-        underlying_source.auto_allocate_chunk_size = TRY(TRY(object.get("autoAllocateChunkSize")).to_bigint_uint64(vm));
+    if (TRY(object.has_property("autoAllocateChunkSize"))) {
+        auto value = TRY(object.get("autoAllocateChunkSize"));
+        underlying_source.auto_allocate_chunk_size = TRY(WebIDL::convert_to_int<WebIDL::UnsignedLongLong>(vm, value, WebIDL::EnforceRange::Yes));
+    }
 
     return underlying_source;
 }