Error in `snapshot()` with packages installed from external source

I have been trying to use packages from my company internal CRAN-like repository in my packrat project. I can install fine, but when I do packrat::snapshot() I get the error

Error: Unable to retrieve package records for the following packages: ...

The problem is that inferPackageRecords sets source = "unknown" for these external packages. Going through the code of getPackageRecords (which is throwing the error), it seems that these packages are picked up by the call to getPackageRecordsExternalSource but then they are dropped because they have unknown source.

To expand further, the error disappears if I explicitly hack the DESCRIPTION file adding Repository:CRAN for example, or if I call getPackageRecords with fallback.ok = TRUE (although there doesn't seem to be a way to do that from snapshot). In both cases the source is (incorrectly) set to "CRAN" and no error is thrown.

Start by tracing inferPackageRecords, getPackageRecords, and getPackageRecordsExternalSource during packrat::snapshot() with a package from an external CRAN-like repository. Compare the behavior with a DESCRIPTION file containing Repository: CRAN and with fallback.ok = TRUE. Done should mean snapshot() handles the external package without a DESCRIPTION hack or incorrectly labeling it as CRAN.