diff --git a/include/csv2/reader.hpp b/include/csv2/reader.hpp
index 45ffb80..45cffea 100644
--- a/include/csv2/reader.hpp
+++ b/include/csv2/reader.hpp
@@ -247,9 +247,10 @@ class Reader {
 
   size_t cols() const {
     size_t result{0};
-    for (const auto cell : header())
+    const auto row = header();
+    for (auto cell = row.begin(); cell != row.end(); ++cell)
       result += 1;
     return result;
   }
 };
-} // namespace csv2
\ No newline at end of file
+} // namespace csv2