diff --git a/src/ruby/pb/test/client.rb b/src/ruby/pb/test/client.rb
index 30550d6..329e2dc 100755
--- a/src/ruby/pb/test/client.rb
+++ b/src/ruby/pb/test/client.rb
@@ -160,7 +160,7 @@
 # produces a string of null chars (\0) of length l.
 def nulls(l)
   fail 'requires #{l} to be +ve' if l < 0
-  [].pack('x' * l).force_encoding('utf-8')
+  [].pack('x' * l).force_encoding('ascii-8bit')
 end
 
 # a PingPongPlayer implements the ping pong bidi test.
diff --git a/src/ruby/pb/test/server.rb b/src/ruby/pb/test/server.rb
index 67877a1..851e815 100755
--- a/src/ruby/pb/test/server.rb
+++ b/src/ruby/pb/test/server.rb
@@ -126,7 +126,7 @@
 # produces a string of null chars (\0) of length l.
 def nulls(l)
   fail 'requires #{l} to be +ve' if l < 0
-  [].pack('x' * l).force_encoding('utf-8')
+  [].pack('x' * l).force_encoding('ascii-8bit')
 end
 
 # A EnumeratorQueue wraps a Queue yielding the items added to it via each_item.