diff --git a/libfprint/drivers/upekts.c b/libfprint/drivers/upekts.c
index 1f1b181..3d82cb3 100644
--- a/libfprint/drivers/upekts.c
+++ b/libfprint/drivers/upekts.c
@@ -366,7 +366,7 @@ read_msg_cb (FpiUsbTransfer *transfer, FpDevice *device,
       fp_err ("async msg read too short (%d)",
               (gint) transfer->actual_length);
       error = fpi_device_error_new_msg (FP_DEVICE_ERROR_PROTO,
-                                        "Packet from device was too short (%lu)",
+                                        "Packet from device was too short (%" G_GSSIZE_FORMAT ")",
                                         transfer->actual_length);
       goto err;
     }
@@ -993,7 +993,7 @@ e_handle_resp00 (FpDevice *dev, unsigned char *data,
 
   if (data_len != 14)
     {
-      fp_err ("received 3001 poll response of %lu bytes?", data_len);
+      fp_err ("received 3001 poll response of %" G_GSIZE_FORMAT " bytes?", data_len);
       do_enroll_stop (dev, NULL,
                       fpi_device_error_new_msg (FP_DEVICE_ERROR_PROTO,
                                                 "received 3001 response with wrong length"));
@@ -1090,7 +1090,7 @@ e_handle_resp02 (FpDevice *dev, unsigned char *data,
 
   if (data_len < sizeof (scan_comp))
     {
-      fp_err ("fingerprint data too short (%lu bytes)", data_len);
+      fp_err ("fingerprint data too short (%" G_GSIZE_FORMAT "u bytes)", data_len);
       error = fpi_device_error_new_msg (FP_DEVICE_ERROR_PROTO, "fingerprint data too short");
     }
   else if (memcmp (data, scan_comp, sizeof (scan_comp)) != 0)
@@ -1318,7 +1318,7 @@ v_handle_resp00 (FpDevice *dev, unsigned char *data,
 
   if (data_len != 14)
     {
-      fp_warn ("received 3001 poll response of %lu bytes?", data_len);
+      fp_warn ("received 3001 poll response of %" G_GSIZE_FORMAT "u bytes?", data_len);
       error = fpi_device_error_new (FP_DEVICE_ERROR_PROTO);
       goto out;
     }