diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index 7fff54b..7bebf45 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -567,7 +567,7 @@ def initialize(addr = '::', family = Socket::AF_UNSPEC)
         raise AddressFamilyError, "unsupported address family: #{family}"
       end
     end
-    prefix, prefixlen = addr.split('/')
+    prefix, prefixlen = addr.split('/', 2)
     if prefix =~ /\A\[(.*)\]\z/i
       prefix = $1
       family = Socket::AF_INET6
diff --git a/test/test_ipaddr.rb b/test/test_ipaddr.rb
index 1aa24eb..281656a 100644
--- a/test/test_ipaddr.rb
+++ b/test/test_ipaddr.rb
@@ -91,6 +91,7 @@ def test_s_new
     assert_raise(IPAddr::InvalidAddressError) { IPAddr.new("[192.168.1.2]/120") }
     assert_raise(IPAddr::InvalidAddressError) { IPAddr.new("[2001:200:300::]\nINVALID") }
     assert_raise(IPAddr::InvalidAddressError) { IPAddr.new("192.168.0.1/32\nINVALID") }
+    assert_raise(IPAddr::InvalidAddressError) { IPAddr.new("192.168.0.1/32/20") }
     assert_raise(IPAddr::InvalidPrefixError) { IPAddr.new("::1/255.255.255.0") }
     assert_raise(IPAddr::InvalidPrefixError) { IPAddr.new("::1/129") }
     assert_raise(IPAddr::InvalidPrefixError) { IPAddr.new("192.168.0.1/33") }