diff --git a/code/X3DImporter.cpp b/code/X3DImporter.cpp
index 1117735..669f46f 100644
--- a/code/X3DImporter.cpp
+++ b/code/X3DImporter.cpp
@@ -95,8 +95,8 @@ struct WordIterator: public std::iterator<std::input_iterator_tag, const char*>
         end_ = other.end_;
         return *this;
     }
-    bool operator==(WordIterator &other) const { return start_ == other.start_; }
-    bool operator!=(WordIterator &other) const { return start_ != other.start_; }
+    bool operator==(const WordIterator &other) const { return start_ == other.start_; }
+    bool operator!=(const WordIterator &other) const { return start_ != other.start_; }
     WordIterator &operator++() {
         start_ += strcspn(start_, whitespace);
         start_ += strspn(start_, whitespace);