diff --git a/include/multipart_parser.hpp b/include/multipart_parser.hpp
index ee028f1..945ebf9 100644
--- a/include/multipart_parser.hpp
+++ b/include/multipart_parser.hpp
@@ -219,7 +219,7 @@
         }
     }
 
-    char lower(char c) const
+    static char lower(char c)
     {
         return static_cast<char>(c | 0x20);
     }