diff --git a/src/ft245r.c b/src/ft245r.c
index 8eab7d6f..5068903a 100644
--- a/src/ft245r.c
+++ b/src/ft245r.c
@@ -276,7 +276,7 @@ static int ft245r_recv(const PROGRAMMER *pgm, unsigned char *buf, size_t len) {
     ft245r_fill(pgm);
 
 #if FT245R_DEBUG
-    msg_info("%s: discarding %d, consuming %zu bytes\n", __func__, rx.discard, len);
+    msg_info("%s: discarding %d, consuming %lu bytes\n", __func__, rx.discard, (unsigned long) len);
 #endif
     while (rx.discard > 0) {
         int result = ft245r_rx_buf_fill_and_get(pgm);
diff --git a/src/jtag3.c b/src/jtag3.c
index 62b95779..470fae75 100644
--- a/src/jtag3.c
+++ b/src/jtag3.c
@@ -2586,7 +2586,7 @@ static int jtag3_send_tpi(const PROGRAMMER *pgm, unsigned char *data, size_t len
 
   cmdbuf[0] = SCOPE_AVR_TPI;
   if (len > INT_MAX) {
-    pmsg_error("invalid jtag3_send_tpi() packet length %zu\n", len);
+    pmsg_error("invalid jtag3_send_tpi() packet length %lu\n", (unsigned long) len);
     free(cmdbuf);
     return -1;
   }