diff --git a/include/hashtables.h b/include/hashtables.h
index a0fbd778b7..2523baa057 100644
--- a/include/hashtables.h
+++ b/include/hashtables.h
@@ -96,7 +96,7 @@ struct fnv_1a
 
         for( ; *it;  ++it )
         {
-            hash ^= *it;
+            hash ^= (unsigned char) *it;
             hash *= 16777619;
         }
         return hash;