Index: src/org/jruby/RubyObject.java
===================================================================
--- src/org/jruby/RubyObject.java	(revision 3663)
+++ src/org/jruby/RubyObject.java	(working copy)
@@ -820,11 +820,15 @@
             public IRubyObject execute(IRubyObject self, IRubyObject[] args, Block block) {
                 IRubyObject source = args[1];
                 IRubyObject filename = args[2];
+                if (!filename.respondsTo("to_str")) {
+                    throw source.getRuntime().newTypeError("can't convert " + filename.getMetaClass() + " into String");
+                }
+                
                 // FIXME: lineNumber is not supported
                 //IRubyObject lineNumber = args[3];
 
-                return args[0].evalSimple(source.getRuntime().getCurrentContext(),
-                                  source, ((RubyString) filename).toString());
+              return args[0].evalSimple(source.getRuntime().getCurrentContext(),
+                      source, filename.convertToString().toString());
             }
 
             public Arity getArity() {