diff --git a/third_party/blink/renderer/platform/wtf/vector.h b/third_party/blink/renderer/platform/wtf/vector.h
index e5ae4f4e50ae1..c11fc662ab26b 100644
--- a/third_party/blink/renderer/platform/wtf/vector.h
+++ b/third_party/blink/renderer/platform/wtf/vector.h
@@ -1876,9 +1876,9 @@ Vector<T, InlineCapacity, Allocator>::Vector(
 template <typename T, wtf_size_t InlineCapacity, typename Allocator>
 template <typename U>
 Vector<T, InlineCapacity, Allocator>::Vector(base::span<const U> other)
-    : Base(other.size()) {
+    : Base(base::checked_cast<wtf_size_t>(other.size())) {
   UNSAFE_TODO(ANNOTATE_NEW_BUFFER(data(), capacity(), other.size()));
-  size_ = other.size();
+  size_ = static_cast<wtf_size_t>(other.size());
   TypeOperations::UninitializedCopy(other, base::span(*this),
                                     VectorOperationOrigin::kConstruction);
 }
@@ -1887,7 +1887,7 @@ template <typename T, wtf_size_t InlineCapacity, typename Allocator>
 template <typename Range, typename Proj>
   requires VectorCanAssignFromRange<T, InlineCapacity, Allocator, Range, Proj>
 Vector<T, InlineCapacity, Allocator>::Vector(Range&& other, Proj proj)
-    : Base(std::ranges::size(other)) {
+    : Base(base::checked_cast<wtf_size_t>(std::ranges::size(other))) {
   // Note that `size(other)` may become smaller if `other` is a hash table
   // with WeakMember keys and `Base(size(other))` above caused GC which
   // removed some entries from `other`, see crbug.com/40448463. This won't