Index: sys/netinet/cc/cc_cubic.h
===================================================================
--- sys/netinet/cc/cc_cubic.h
+++ sys/netinet/cc/cc_cubic.h
@@ -184,6 +184,9 @@
 	 */
 	cwnd = ((cwnd * CUBIC_C_FACTOR * smss) >> CUBIC_SHIFT_4) + wmax;
 
+	if (cwnd < (int64_t)smss)
+		return (unsigned long)smss;
+
 	return ((unsigned long)cwnd);
 }