diff --git a/include/http_utility.hpp b/include/http_utility.hpp
index 740e218..3970992 100644
--- a/include/http_utility.hpp
+++ b/include/http_utility.hpp
@@ -53,7 +53,7 @@
     for (const char c : value)
     {
         // Keep alphanumeric and other accepted characters intact
-        if (isalnum(c) || c == '-' || c == '_' || c == '.' || c == '~')
+        if ((isalnum(c) != 0) || c == '-' || c == '_' || c == '.' || c == '~')
         {
             escaped << c;
             continue;