diff --git a/test/runner-unix.c b/test/runner-unix.c
index 61d6a96d..fa4c63bb 100644
--- a/test/runner-unix.c
+++ b/test/runner-unix.c
@@ -248,7 +248,7 @@ int process_copy_output(process_info_t *p, int fd) {
     return -1;
   }
 
-  size_t nread, nwritten;
+  ssize_t nread, nwritten;
   char buf[1024];
 
   while ((nread = read(fileno(p->stdout_file), buf, 1024)) > 0) {