diff --git a/src/utils.cpp b/src/utils.cpp
index 9aab9ed7e..d6096aec9 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -237,7 +237,7 @@ ustring &ustring::operator = (const ustring &s)
 } // ustring & ustring::operator = (const ustring &s)
 
 // Compare two substrings
-bool ustring::operator == (const ustring &s)
+bool ustring::operator == (const ustring &s) const
 {
     size_t len = length();
     if (s.length() != len)
diff --git a/src/utils.h b/src/utils.h
index e7bc09932..2b03261f5 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -312,7 +312,7 @@ class ustring
 
     ustring &operator = (const ustring &s);
 
-    bool operator == (const ustring &s);
+    bool operator == (const ustring &s) const;
 
     uint16 &operator [](size_t pos) {
         return _str[pos];