diff --git a/include/jwt/base64.hpp b/include/jwt/base64.hpp
index 1f2e921..e3baf6e 100644
--- a/include/jwt/base64.hpp
+++ b/include/jwt/base64.hpp
@@ -159,7 +159,7 @@ public:
   }
 
 private:
-  std::array<char, 256> map_ = {{
+  std::array<signed char, 256> map_ = {{
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, //   0-15
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, //  16-31
     -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, //  32-47