diff --git a/source/cell/cell.cpp b/source/cell/cell.cpp
index f942afcb..22b10f49 100644
--- a/source/cell/cell.cpp
+++ b/source/cell/cell.cpp
@@ -376,7 +376,6 @@ void cell::hyperlink(const std::string &url)
 
     auto ws = worksheet();
     auto &manifest = ws.workbook().manifest();
-    bool existing = false;
 
     d_->hyperlink_ = detail::hyperlink_impl();
 
diff --git a/source/detail/serialization/custom_value_traits.hpp b/source/detail/serialization/custom_value_traits.hpp
index c829d518..9f33d08d 100644
--- a/source/detail/serialization/custom_value_traits.hpp
+++ b/source/detail/serialization/custom_value_traits.hpp
@@ -198,7 +198,11 @@ pattern_fill_type from_string(const std::string &string)
     auto toLower = [](std::string str) {
         auto bg{ std::begin (str) };
         auto en{ std::end (str) };
-        std::transform (bg, en, bg, tolower);
+        std::transform(bg, en, bg, 
+                       [](char c) {
+                        // static cast to avoid int -> char narrowing warning
+                           return static_cast<char>(tolower(c)); 
+                       });
 
         return str;
     };
diff --git a/source/detail/serialization/xlsx_consumer.cpp b/source/detail/serialization/xlsx_consumer.cpp
index 854263e8..8d6c922f 100644
--- a/source/detail/serialization/xlsx_consumer.cpp
+++ b/source/detail/serialization/xlsx_consumer.cpp
@@ -246,7 +246,7 @@ cell xlsx_consumer::read_cell()
 
     if (parser().attribute_present("s"))
     {
-        cell.format(target_.format(std::stoull(parser().attribute("s"))));
+        cell.format(target_.format(static_cast<std::size_t>(std::stoull(parser().attribute("s")))));
     }
 
     auto has_value = false;
@@ -673,7 +673,7 @@ void xlsx_consumer::read_worksheet_sheetdata()
 
             if (parser().attribute_present("s"))
             {
-		            cell.format(target_.format(std::stoull(parser().attribute("s"))));
+                cell.format(target_.format(static_cast<std::size_t>(std::stoull(parser().attribute("s")))));
             }
 
             auto has_value = false;