diff --git a/src/manifest.jl b/src/manifest.jl
index 45fcdd06c0..e1e65a9946 100644
--- a/src/manifest.jl
+++ b/src/manifest.jl
@@ -131,7 +131,7 @@ function validate_manifest(julia_version::Union{Nothing,VersionNumber}, manifest
 end
 
 function Manifest(raw::Dict, f_or_io::Union{String, IO})::Manifest
-    julia_version = isnothing(raw["julia_version"]) ? nothing : VersionNumber(raw["julia_version"])
+    julia_version = raw["julia_version"] == "nothing" ? nothing : VersionNumber(raw["julia_version"])
     manifest_format = VersionNumber(raw["manifest_format"])
     if !in(manifest_format.major, 1:2)
         if f_or_io isa IO
@@ -201,7 +201,7 @@ end
 function convert_flat_format_manifest(old_raw_manifest::Dict)
     new_raw_manifest = Dict{String, Any}(
             "deps" => old_raw_manifest,
-            "julia_version" => nothing,
+            "julia_version" => "nothing", # must be a string here to match raw dict
             "manifest_format" => "1.0.0" # must be a string here to match raw dict
         )
     return new_raw_manifest
diff --git a/test/manifests.jl b/test/manifests.jl
index 30a792bb99..fe369c13e0 100644
--- a/test/manifests.jl
+++ b/test/manifests.jl
@@ -86,6 +86,35 @@ using  ..Utils
             @test_logs (:warn,) Pkg.activate(env_dir; io=io)
         end
     end
+
+    @testset "Pkg.upgrade_manifest()" begin
+        env_dir = joinpath(@__DIR__, "manifest", "formats", "v1.0")
+        env_manifest = joinpath(env_dir, "Manifest.toml")
+        cp(env_manifest, string(env_manifest, "_backup"))
+        try
+            isfile(env_manifest) || error("Reference manifest is missing")
+            if Base.is_v1_format_manifest(Base.parsed_toml(env_manifest)) == false
+                error("Reference manifest file at $(env_manifest) is invalid")
+            end
+            isolate(loaded_depot=true) do
+                io = IOBuffer()
+                Pkg.activate(env_dir; io=io)
+                output = String(take!(io))
+                @test occursin(r"Activating.*project at.*`.*v1.0`", output)
+                @test Base.is_v1_format_manifest(Base.parsed_toml(env_manifest))
+
+                Pkg.upgrade_manifest()
+                @test Base.is_v1_format_manifest(Base.parsed_toml(env_manifest)) == false
+                Pkg.activate(env_dir; io=io)
+                output = String(take!(io))
+                @test occursin(r"Activating.*project at.*`.*v1.0`", output)
+                @test Pkg.Types.Context().env.manifest.manifest_format == v"2.0.0"
+            end
+        finally
+            cp(string(env_manifest, "_backup"), env_manifest, force = true)
+            rm(string(env_manifest, "_backup"))
+        end
+    end
 end