IThenable and non-nullable types

When trying to tighten up the types in my code, I'm getting a number of
warnings that seem spurious e.g.

foo.js:152: WARNING - actual parameter 1 of foo does not match formal
parameter
found : (IThenable.<Object.<string,?>>|null)
required: (IThenable.<(Object.<string,?>|null)>|null)
foo(factory.getVersions(this.params))

Where the actual type (a promise for a non-null object) is stricter than
the requirement (a promise for a nullable object).

Similarly I've run into issues trying objects as @type {!IThenable.<XYZ>},
because IThenable.prototype.then seems to return a nullable type, but I
don't think promises actually return null values from then(). Are there
any good ways to avoid these problems?

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