diff --git a/src/rebar_app_info.erl b/src/rebar_app_info.erl
index 15b9701d..9b6fba42 100644
--- a/src/rebar_app_info.erl
+++ b/src/rebar_app_info.erl
@@ -90,7 +90,7 @@
                      app_file           :: file:filename_all() | undefined,
                      original_vsn       :: app_vsn() | undefined,
                      vsn                :: app_vsn() | undefined,
-                     parent=root        :: binary() | root | string(),
+                     parent=root        :: binary() | root,
                      app_details=[]     :: list(),
                      applications=[]    :: list(),
                      included_applications=[] :: [atom()],
@@ -415,12 +415,12 @@ app_details(AppInfo=#app_info_t{}, AppDetails) ->
     AppInfo#app_info_t{app_details=AppDetails}.
 
 %% @doc returns the app's parent in the dep tree.
--spec parent(t()) -> root | binary() | string().
+-spec parent(t()) -> root | binary().
 parent(#app_info_t{parent=Parent}) ->
     Parent.
 
 %% @doc sets the app's parent.
--spec parent(t(), binary() | root | string()) -> t().
+-spec parent(t(), binary() | root) -> t().
 parent(AppInfo=#app_info_t{}, Parent) ->
     AppInfo#app_info_t{parent=Parent}.
 
diff --git a/src/rebar_app_utils.erl b/src/rebar_app_utils.erl
index 64a40c46..4786101f 100644
--- a/src/rebar_app_utils.erl
+++ b/src/rebar_app_utils.erl
@@ -171,7 +171,7 @@ parse_deps(Parent, DepsDir, Deps, State, Locks, Level) ->
 %% (if it is a newer thing) or from the locks specified in the lockfile.
 -spec parse_dep(Dep, Parent, Dir, State, Locks, Level) -> rebar_app_info:t() when
       Dep :: tuple() | atom() | binary(), % TODO: meta to source() | lock()
-      Parent :: root | binary() | string(),
+      Parent :: root | binary(),
       Dir :: file:filename(),
       State :: rebar_state:t(),
       Locks :: [tuple()], % TODO: meta to [lock()]
@@ -199,7 +199,7 @@ parse_dep(Dep, Parent, DepsDir, State, Locks, Level) ->
 %% @doc converts a dependency definition and a location for it on disk
 %% into an app info tuple representing it.
 -spec parse_dep(Parent, Dep, Dir, IsLock, State) -> rebar_app_info:t() when
-      Parent :: root | binary() | string(),
+      Parent :: root | binary(),
       Dep :: tuple() | atom() | binary(), % TODO: meta to source() | lock()
       Dir :: file:filename(),
       IsLock :: boolean(),
@@ -244,7 +244,7 @@ parse_dep(_, Dep, _, _, _) ->
 %% @doc convert a dependency that has just been fetched into
 %% an app info record related to it
 -spec dep_to_app(Parent, Dir, Name, Vsn, Source, IsLock, State) -> rebar_app_info:t() when
-      Parent :: root | binary() | string(),
+      Parent :: root | binary(),
       Dir :: file:filename(),
       Name :: binary(),
       Vsn :: iodata() | undefined,
diff --git a/src/rebar_prv_plugins_upgrade.erl b/src/rebar_prv_plugins_upgrade.erl
index c62876c7..3ad3dc72 100644
--- a/src/rebar_prv_plugins_upgrade.erl
+++ b/src/rebar_prv_plugins_upgrade.erl
@@ -97,7 +97,7 @@ build_plugin(ToBuild, State) ->
 maybe_update_pkg(Tup, State) when is_tuple(Tup) ->
     maybe_update_pkg(element(1, Tup), State);
 maybe_update_pkg(Name, State) ->
-    try rebar_app_utils:parse_dep(root, ?DEFAULT_PLUGINS_DIR, Name, State, [], 0) of
+    try rebar_app_utils:parse_dep(root, unicode:characters_to_binary(?DEFAULT_PLUGINS_DIR), Name, State, [], 0) of
         AppInfo ->
             Source = rebar_app_info:source(AppInfo),
             case element(1, Source) of