Index: ironruby/Runtime/RubyExecutionContext.cs
===================================================================
--- ironruby/Runtime/RubyExecutionContext.cs	(revision 76)
+++ ironruby/Runtime/RubyExecutionContext.cs	(working copy)
@@ -202,9 +202,9 @@
         private void InitializeGlobalConstants() {
             Debug.Assert(_objectClass != null);
 
-            string version = RubyContext.Version;
+            MutableString version = new MutableString(RubyContext.Version);
             MutableString platform = new MutableString("i386-mswin32");   // TODO: make this the correct string for MAC OS X in Silverlight
-            string releaseDate = RubyContext.ReleaseDate;
+            MutableString releaseDate = new MutableString(RubyContext.ReleaseDate);
 
             _objectClass.SetConstant("RUBY_VERSION", version);
             _objectClass.SetConstant("RUBY_PLATFORM", platform);