diff --git a/src/hashset.hpp b/src/hashset.hpp
index 16584ebe..13b6bf7f 100644
--- a/src/hashset.hpp
+++ b/src/hashset.hpp
@@ -30,7 +30,7 @@ vt_hash_const_string(const char *key_)
   uint64_t hash = 0xcbf29ce484222325ull;
 
   while( *key_ )
-    hash = ( (unsigned char)*key++ ^ hash ) * 0x100000001b3ull;
+    hash = ((const unsigned char)*key_++ ^ hash ) * 0x100000001b3ull;
 
   return hash;  
 }