diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 0d97aa45fbc..09dea4bbe16 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -928,6 +928,8 @@ pq_getbyte(void)
 int
 pq_peekbyte(void)
 {
+	Assert(PqCommReadingMsg);
+
 	while (PqRecvPointer >= PqRecvLength)
 	{
 		if (pq_recvbuf())		/* If nothing in buffer, then recv some */