diff --git a/src/xcb_in.c b/src/xcb_in.c
index 31a1e60..1d029af 100644
--- a/src/xcb_in.c
+++ b/src/xcb_in.c
@@ -252,7 +252,7 @@ static void free_reply_list(struct reply_list *head)
     }
 }
 
-static int read_block(const int fd, void *buf, const size_t len)
+static int read_block(const int fd, void *buf, const ssize_t len)
 {
     int done = 0;
     while(done < len)