diff --git a/dotnet/compiler/Actions.pm b/dotnet/compiler/Actions.pm
index 2be0f7e..b0b0649 100644
--- a/dotnet/compiler/Actions.pm
+++ b/dotnet/compiler/Actions.pm
@@ -687,6 +687,11 @@ method parameter($/) {
         $past.multitype($<typename>[0].ast);
     }
 
+    # Set definedness flag (XXX perhaps want a better way to do this).
+    if $<definedness> {
+        $past<definedness> := ~$<definedness>[0];
+    }
+
     make $past;
 }
 
diff --git a/dotnet/compiler/Grammar.pm b/dotnet/compiler/Grammar.pm
index ef7d946..6bfe21e 100644
--- a/dotnet/compiler/Grammar.pm
+++ b/dotnet/compiler/Grammar.pm
@@ -371,7 +371,7 @@ token multi_declarator:sym<null> {
 token signature { [ [<.ws><parameter><.ws>] ** ',' ]? }
 
 token parameter {
-    [ <typename> <.ws> ]*                   # <type_constraint>
+    [ <typename> [ ':' $<definedness>=<[_DU]> ]? <.ws> ]*                   # <type_constraint>
     [
     | $<quant>=['*'] <param_var>
     | [ <param_var> | <named_param> ] $<quant>=['?'|'!'|<?>]