When type checking a method (or new) expression Byteman has to filter candidate methods (or, respectively, constructors) to pick the target for the call, by reconciling the type signature of each candidate with the type information available for the actual method arguments. It is supposed to pick the method lowest down the tree whose signature types are compatible with the argument types. Compatible here means that the actual type equals the parameter type modulo conversion of the actual type either by upcasting or by one of the automatic type conversions Byteman is willing to perform (i.e. boxing, unboxing, toString conversion or numeric interconversion). This check appears to be failing in the case of a new expression (at least) as exemplified in the related forum post where an upcast from actual argument type FileOutputStream to expected parameter type OutputStream is not being performed.