How to construct a block comment from text (Java)?

I am developing a plugin in which I want to rewrite the body of a method. For example, I want to do something like this:

Before rewriting:

int f(){
  int x = 2;
  return x;
}

After rewriting:

int f(){
  int y = 3;
  return y;
  /*int x = 2;
  return x;*/
}

As you can see, I want to (1) modify the method body (2) create a comment out of the old method body text and (3) add it in the rewritten method. I constructed the comment as follows:

PsiCommentImpl oldBodyComment = new PsiCommentImpl(JavaTokenType.C_STYLE_COMMENT, oldBodyText);

but when I try to do (3):

newMethodBody.add(oldBodyComment); //I am running this as a Runnable using runWriteCommandAction

it gives me the following error:

com.intellij.psi.PsiInvalidElementAccessException: Element: class com.intellij.psi.impl.source.tree.PsiCommentImpl because: parent is null;

What am I doing wrong? In general, what is the correct way to construct comments from some text in an IntelliJ IDEA plugin?

Any help is much appreciated.