diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index 7a5cf94..5cab9d4 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -652,7 +652,7 @@ def in_addr(addr)
     end
     octets.inject(0) { |i, s|
       (n = s.to_i) < 256 or raise InvalidAddressError, "invalid address: #{@addr}"
-      s.match(/\A0./) and raise InvalidAddressError, "zero-filled number in IPv4 address is ambiguous: #{@addr}"
+      (s != '0') && s.start_with?('0') and raise InvalidAddressError, "zero-filled number in IPv4 address is ambiguous: #{@addr}"
       i << 8 | n
     }
   end