diff --git a/include/ftl/concat.h b/include/ftl/concat.h
index e0774d3..7680bed 100644
--- a/include/ftl/concat.h
+++ b/include/ftl/concat.h
@@ -57,7 +57,7 @@
 template <std::size_t N>
 struct Concat<N> {
   static constexpr std::size_t max_size() { return N; }
-  constexpr std::size_t size() const { return end_ - buffer_; }
+  constexpr std::size_t size() const { return static_cast<std::size_t>(end_ - buffer_); }
 
   constexpr const char* c_str() const { return buffer_; }
 
@@ -68,6 +68,8 @@
 
  protected:
   constexpr Concat() : end_(buffer_) {}
+  constexpr Concat(const Concat&) = delete;
+
   constexpr void append() { *end_ = '\0'; }
 
   char buffer_[N + 1];