diff --git a/src/curl.h b/src/curl.h
index 84d05d4..0b164a2 100644
--- a/src/curl.h
+++ b/src/curl.h
@@ -85,7 +85,7 @@ struct filepart
 // for progress
 struct case_insensitive_compare_func
 {
-  bool operator()(const std::string& a, const std::string& b){
+  bool operator()(const std::string& a, const std::string& b) const {
     return strcasecmp(a.c_str(), b.c_str()) < 0;
   }
 };