diff --git a/clock.c b/clock.c
index 459b7b9..eeefd39 100644
--- a/clock.c
+++ b/clock.c
@@ -402,7 +402,9 @@ struct clock *clock_create(int phc_index, struct interface *iface, int count,
 	if (c->nports)
 		clock_destroy(c);
 
-	if (phc_index >= 0) {
+	if (c->dds.free_running) {
+		c->clkid = CLOCK_INVALID;
+	} else if (phc_index >= 0) {
 		snprintf(phc, 31, "/dev/ptp%d", phc_index);
 		c->clkid = phc_open(phc);
 		if (c->clkid == CLOCK_INVALID) {
diff --git a/ptp4l.c b/ptp4l.c
index 12bb3f3..6fc210a 100644
--- a/ptp4l.c
+++ b/ptp4l.c
@@ -247,7 +247,9 @@ int main(int argc, char *argv[])
 	}
 
 	/* determine PHC Clock index */
-	if (*timestamping == TS_SOFTWARE || *timestamping == TS_LEGACY_HW) {
+	if (ds->free_running) {
+		phc_index = -1;
+	} else if (*timestamping == TS_SOFTWARE || *timestamping == TS_LEGACY_HW) {
 		phc_index = -1;
 	} else if (req_phc) {
 		if (1 != sscanf(req_phc, "/dev/ptp%d", &phc_index)) {