diff --git a/include/wx/string.h b/include/wx/string.h
index 504f0fd325..c15a80f0cc 100644
--- a/include/wx/string.h
+++ b/include/wx/string.h
@@ -1041,7 +1041,7 @@ public:
       reverse_iterator_impl operator-=(ptrdiff_t n)
         { m_cur += n; return *this; }
 
-      unsigned operator-(const reverse_iterator_impl& i) const
+      difference_type operator-(const reverse_iterator_impl& i) const
         { return i.m_cur - m_cur; }
 
       bool operator==(const reverse_iterator_impl& ri) const