Returning an ImmutableMap <File, File>

I have a method that returns a Map. I would initially return the HashMap that the method generated, but thought it would be better to return an ImmutableMap. Unfortunately, the following statement refuses to work in eclipse:

HashMap<File, File> map = new HashMap<File, File>();
map.put(...);
.
.
.
return ImmutableMap.builder ().putAll (map).build ();

It keeps saying that I'm returning an incompatible statement, a Map<Object, Object>.

I initially tried to use:

return ImmutableMap<File, File>.builder ().putAll (map).build ();

but that obviously didn't work. How would I best go about fixing this? Should I first store it in something like

ImmutableMap<File, File> m = ImmutableMap.builder ().putAll (map).build ();

or is there a more elegant solution?