diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index 525466b..1ad7980 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -381,7 +381,9 @@ def ipv4_compat
     if !ipv4?
       raise InvalidAddressError, "not an IPv4 address: #{@addr}"
     end
-    return self.clone.set(@addr, Socket::AF_INET6)
+    clone = self.clone.set(@addr, Socket::AF_INET6)
+    clone.instance_variable_set(:@mask_addr, @mask_addr | 0xffffffffffffffffffffffff00000000)
+    clone
   end
 
   # Returns a new ipaddr built by converting the IPv6 address into a
diff --git a/test/test_ipaddr.rb b/test/test_ipaddr.rb
index 7ecd37e..ac8921e 100644
--- a/test/test_ipaddr.rb
+++ b/test/test_ipaddr.rb
@@ -196,6 +196,13 @@ def test_ipv4_compat
     }
     assert_equal("::192.168.1.2", b.to_s)
     assert_equal(Socket::AF_INET6, b.family)
+    assert_equal(128, b.prefix)
+
+    a = IPAddr.new("192.168.0.0/16")
+    b = a.ipv4_compat
+    assert_equal("::192.168.0.0", b.to_s)
+    assert_equal(Socket::AF_INET6, b.family)
+    assert_equal(112, b.prefix)
   end
 
   def test_ipv4_mapped