diff --git a/src/prevector.h b/src/prevector.h
index dc17e7ce4..02d860bb0 100644
--- a/src/prevector.h
+++ b/src/prevector.h
@@ -132,7 +132,7 @@ public:
         typedef const T* pointer;
         typedef const T& reference;
         typedef std::bidirectional_iterator_tag iterator_category;
-        const_reverse_iterator(T* ptr_) : ptr(ptr_) {}
+        const_reverse_iterator(const T* ptr_) : ptr(ptr_) {}
         const_reverse_iterator(reverse_iterator x) : ptr(&(*x)) {}
         const T& operator*() const { return *ptr; }
         const T* operator->() const { return ptr; }