diff --git a/lib/conncache.h b/lib/conncache.h
index 691f061f94..866554b63b 100644
--- a/lib/conncache.h
+++ b/lib/conncache.h
@@ -25,7 +25,7 @@
 struct conncache {
   struct curl_hash *hash;
   size_t num_connections;
-  size_t next_connection_id;
+  long next_connection_id;
 };
 
 struct conncache *Curl_conncache_init(int size);