diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index beb1c889aad..551276e9011 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -1544,7 +1544,11 @@ getBackendKeyData(PGconn *conn, int msgLength)
 	if (pqGetInt(&(conn->be_pid), 4, conn))
 		return EOF;
 
-	cancel_key_len = 5 + msgLength - (conn->inCursor - conn->inStart);
+	/*
+	 * The pid consumed 4 bytes, and the cancellation key consumes the rest of
+	 * the message.
+	 */
+	cancel_key_len = msgLength - 4;
 
 	conn->be_cancel_key = malloc(cancel_key_len);
 	if (conn->be_cancel_key == NULL)