Attempt to fix issue #2942

src/Operations.jl

@@ -428,7 +428,7 @@ function deps_graph(env::EnvCache, registries::Vector{Registry.RegistryInstance}
             # unregistered stdlib we must special-case it here.  This is further
             # complicated by the fact that we can ask this question relative to
             # a Julia version.
-            if is_unregistered_stdlib(uuid) || uuid_is_stdlib
+            if (is_unregistered_stdlib(uuid) || uuid_is_stdlib) && haskey(stdlibs(), uuid)
                 path = Types.stdlib_path(stdlibs_for_julia_version[uuid][1])
                 proj_file = projectfile_path(path; strict=true)
                 @assert proj_file !== nothing

test/resolve.jl

@@ -505,6 +505,11 @@ end
         mpfr = find_by_name(versions, "MPFR_jll")
         @test mpfr !== nothing
         @test mpfr.version.major == 4 && mpfr.version.minor == 0
+
+        # Test for issue #2942
+        libjulia_jll = "libjulia_jll"
+        Pkg.add(Pkg.Types.Context(; julia_version = v"1.6"), [PackageSpec(; name = libjulia_jll)])
+        @test isinstalled(libjulia_jll)
     end
 end