diff --git a/mx_proc.c b/mx_proc.c
index 48e590cc..4ffbaa70 100644
--- a/mx_proc.c
+++ b/mx_proc.c
@@ -18,7 +18,7 @@ static long long int get_rss_anon(pid_t pid) {
     _mx_cleanup_free_ char *buf = NULL;
     size_t n = 0;
     while(1) {
-        size_t len = getline(&buf, &n, file);
+        ssize_t len = getline(&buf, &n, file);
         if (len == -1)
             break;
         if (strncmp(buf, "RssAnon:", 8) == 0) {