error in scapy attribute 'haslayer'

[evilcode1]

hello all ...
im trying to write a port scanner using scapy module
my code :

from scapy.all import * 

target = "192.168.1.81"


print("Scanning .... " + target + " For Open Ports!\n")

for x in range(20 , 100):
    packet = IP(dst=target)/TCP(dport=x,flags='S')
    res = sr1(packet ,timeout=0.5)
    if res.haslayer(TCP) and res.getlayer(TCP).flags==0x12:
        print("port" + str(x) + "is open!")
    sr(IP(dst=target)/TCP(dport=res.sport,flags='R'),timeout=0.5)
print("scan is complete")

Output:
Scanning .... 192.168.1.81 For Open Ports!

Begin emission:
Finished sending 1 packets.
.....
Received 5 packets, got 0 answers, remaining 1 packets
Traceback (most recent call last):
  File "/home/evilcode1/Downloads/Course/ping.py", line 11, in <module>
    if res.haslayer(TCP) and res.getlayer(TCP).flags==0x12:
AttributeError: 'NoneType' object has no attribute 'haslayer'

any help please