diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index d4218550a..587882883 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -72,11 +72,13 @@ static PathSet realisePath(StorePathWithOutputs path, bool build = true)
         Derivation drv = store->derivationFromPath(path.path);
         rootNr++;
 
+        /* FIXME: Encode this empty special case explicitly in the type. */
         if (path.outputs.empty())
             for (auto & i : drv.outputs) path.outputs.insert(i.first);
 
         PathSet outputs;
         for (auto & j : path.outputs) {
+            /* Match outputs of a store path with outputs of the derivation that produces it. */
             DerivationOutputs::iterator i = drv.outputs.find(j);
             if (i == drv.outputs.end())
                 throw Error("derivation '%s' does not have an output named '%s'",
@@ -141,6 +143,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
         toDerivedPaths(paths),
         willBuild, willSubstitute, unknown, downloadSize, narSize);
 
+    /* Filter out unknown paths from `paths`. */
     if (ignoreUnknown) {
         std::vector<StorePathWithOutputs> paths2;
         for (auto & i : paths)