diff --git a/src/ReTestItems.jl b/src/ReTestItems.jl
index 00342ebe..a6ff0beb 100644
--- a/src/ReTestItems.jl
+++ b/src/ReTestItems.jl
@@ -633,17 +633,19 @@ function is_testsetup_file(filepath)
     )
 end
 
-# like `relpath` but assumes `path` is nested under `startdir`, else just returns `path`
-function nestedrelpath(path, startdir)
-    path == startdir && return "."
+# Like `relpath` but assumes `path` is nested under `startdir`, else just returns `path`.
+# Always returns a `SubString` to be type-stable.
+function nestedrelpath(path::T, startdir::AbstractString) where {T <: AbstractString}
+    path == startdir && return SubString{T}(".")
     relp = chopprefix(path, startdir)
+    relp == path && return relp
     sep = Base.Filesystem.path_separator
     if endswith(startdir, sep)
         return relp
     elseif startswith(relp, sep)
         return chopprefix(relp, sep)
-    else
-        return path
+    else # `startdir` was a prefix of `path` but not a directory
+        return SubString{T}(path)
     end
 end
 
diff --git a/test/internals.jl b/test/internals.jl
index 0b1e7b89..ccfc4d38 100644
--- a/test/internals.jl
+++ b/test/internals.jl
@@ -394,13 +394,11 @@ end
     @test nestedrelpath(path, "test/dir/other") == "test/dir/foo_test.jl"
     @test nestedrelpath(path, "test/dir/other/bar_test.jl") == "test/dir/foo_test.jl"
 
-    @static if isdefined(Base, Symbol("@allocations")) # added in Julia v1.9
-        @test 2 >= @allocations(nestedrelpath(path, "test"))
-        @test 2 >= @allocations(nestedrelpath(path, "test/dir"))
-        @test 1 >= @allocations(nestedrelpath(path, "test/dir/foo_test.jl"))
-        @test 1 >= @allocations(nestedrelpath(path, "test/dir/other"))
-        @test 1 >= @allocations(nestedrelpath(path, "test/dir/other/bar_test.jl"))
-    end
+    # leading '/' doesn't get ignored or stripped
+    @test nestedrelpath("/a/b/c", "/a/b") == "c"
+    @test nestedrelpath("/a/b/c", "a/b") == "/a/b/c"
+    @test nestedrelpath("/a/b", "/a/b/c") == "/a/b"
+    @test nestedrelpath("/a/b", "c") == "/a/b"
 end
 
 end # internals.jl testset