diff --git a/phc2sys.8 b/phc2sys.8
index adce4ad..a5051c8 100644
--- a/phc2sys.8
+++ b/phc2sys.8
@@ -54,7 +54,8 @@ should be already close to the correct time before
 .B phc2sys
 is started or the
 .B \-s
-option should be used too. 
+option should be used too. This option can be used only with the system clock as
+the slave clock. 
 .TP
 .BI \-s " phc-device"
 Specify the master clock by device (e.g. /dev/ptp0) or name (e.g. CLOCK_REALTIME
diff --git a/phc2sys.c b/phc2sys.c
index 1c0d8b2..988a3f0 100644
--- a/phc2sys.c
+++ b/phc2sys.c
@@ -378,7 +378,8 @@ int main(int argc, char *argv[])
 		src = clock_open(phc_device);
 	}
 	if (!(device || src != CLOCK_INVALID) ||
-	    dst_clock.clkid == CLOCK_INVALID) {
+	    dst_clock.clkid == CLOCK_INVALID ||
+	    (device && dst_clock.clkid != CLOCK_REALTIME)) {
 		usage(progname);
 		return -1;
 	}