diff --git a/src/client/user_simtrace2.c b/src/client/user_simtrace2.c
index ea89a82..39e6b32 100644
--- a/src/client/user_simtrace2.c
+++ b/src/client/user_simtrace2.c
@@ -411,6 +411,7 @@
 	ifm->addr = cfg->usb.addr;
 	if (cfg->usb.path)
 		osmo_strlcpy(ifm->path, cfg->usb.path, sizeof(ifm->path));
+	transp->udp_fd = -1;
 	transp->usb_devh = osmo_libusb_open_claim_interface(NULL, NULL, ifm);
 	if (!transp->usb_devh) {
 		fprintf(stderr, "can't open USB device\n");