diff --git a/lib/ipaddress/prefix.rb b/lib/ipaddress/prefix.rb
index f3d585d65ba2f2e362a23c0a1caa103e2b703e0f..a0e3bb0914f543b07c391c9cd63bc75c95650de4 100644
--- a/lib/ipaddress/prefix.rb
+++ b/lib/ipaddress/prefix.rb
@@ -214,8 +214,8 @@ module IPAddress
     #     #=> 64
     #
     def initialize(num=128)
-      unless (1..128).include? num.to_i
-        raise ArgumentError, "Prefix must be in range 1..128, got: #{num}"
+      unless (0..128).include? num.to_i
+        raise ArgumentError, "Prefix must be in range 0..128, got: #{num}"
       end
       super(num.to_i)
     end