diff --git a/duff.c b/duff.c
--- a/duff.c
+++ b/duff.c
@@ -8,9 +8,9 @@
 #include <stdlib.h>
 #include <string.h>
 
-char *duffcopy (char *to, char *from, int len)
+char *duffcopy (char *to, char *from, size_t len)
 {
-    int n, pos = 0;
+    size_t n, pos = 0;
 
     n = (len + 7) / 8;
 
@@ -49,7 +49,7 @@
 int main (int argc, char **argv)
 {
     char *to, *from;
-    int len;
+    size_t len;
 
     if (argc != 2) {
         printf ("Usage: %s <string>\n", argv[0]);