diff --git a/conffile.cpp b/conffile.cpp
index 817afae7..fa7de3fa 100644
--- a/conffile.cpp
+++ b/conffile.cpp
@@ -452,7 +452,7 @@ void ConfigFile::ClearLines()
     }
 }
 
-bool ConfigFile::ConfigEntry::section_then_key_less::operator()(const ConfigEntry &a, const ConfigEntry &b) {
+bool ConfigFile::ConfigEntry::section_then_key_less::operator()(const ConfigEntry &a, const ConfigEntry &b) const{
 	if(curConfigFile && a.section!=b.section){
 		const int sva = curConfigFile->GetSectionSize(a.section);
 		const int svb = curConfigFile->GetSectionSize(b.section);
diff --git a/conffile.h b/conffile.h
index ba69812d..d92128cb 100644
--- a/conffile.h
+++ b/conffile.h
@@ -90,7 +90,7 @@ class ConfigFile {
 		mutable bool used;
 
         struct section_then_key_less {
-            bool operator()(const ConfigEntry &a, const ConfigEntry &b);
+            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const;
         };
 
         struct key_less {
@@ -101,8 +101,8 @@ class ConfigFile {
         };
 
         struct line_less {
-            bool operator()(const ConfigEntry &a, const ConfigEntry &b){
-				if(a.line==b.line) return (b.val.empty() && !a.val.empty()) || a.key<b.key;
+            bool operator()(const ConfigEntry &a, const ConfigEntry &b) const{
+                if(a.line==b.line) return (b.val.empty() && !a.val.empty()) || a.key<b.key;
                 if(b.line<0) return true;
                 if(a.line<0) return false;
                 return a.line<b.line;