diff --git a/ts2phc_slave.c b/ts2phc_slave.c
index 71077a7..749efe5 100644
--- a/ts2phc_slave.c
+++ b/ts2phc_slave.c
@@ -7,6 +7,7 @@
 #include <errno.h>
 #include <linux/ptp_clock.h>
 #include <poll.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <sys/ioctl.h>
@@ -50,7 +51,7 @@ struct ts2phc_slave_array {
 
 struct ts2phc_source_timestamp {
 	struct timespec ts;
-	int valid;
+	bool valid;
 };
 
 enum extts_result {
@@ -269,6 +270,11 @@ static int ts2phc_slave_event(struct ts2phc_slave *slave,
 		return 0;
 	}
 
+	if (!source_ts.valid) {
+		pr_debug("%s ignoring invalid master time stamp", slave->name);
+		return 0;
+	}
+
 	adj = servo_sample(slave->servo, offset, extts_ts,
 			   SAMPLE_WEIGHT, &slave->state);
 
@@ -398,7 +404,7 @@ int ts2phc_slave_poll(struct ts2phc_master *master)
 {
 	struct ts2phc_source_timestamp source_ts;
 	unsigned int i;
-	int cnt;
+	int cnt, err;
 
 	if (ts2phc_slave_array_create()) {
 		return -1;
@@ -416,7 +422,8 @@ int ts2phc_slave_poll(struct ts2phc_master *master)
 		return 0;
 	}
 
-	source_ts.valid = ts2phc_master_getppstime(master, &source_ts.ts);
+	err = ts2phc_master_getppstime(master, &source_ts.ts);
+	source_ts.valid = err ? false : true;
 
 	for (i = 0; i < ts2phc_n_slaves; i++) {
 		if (polling_array.pfd[i].revents & (POLLIN|POLLPRI)) {