diff --git a/src/rebar_plugins.erl b/src/rebar_plugins.erl
index 2a78c6eb..43b33db8 100644
--- a/src/rebar_plugins.erl
+++ b/src/rebar_plugins.erl
@@ -131,7 +131,7 @@ handle_plugin(Profile, Plugin, State, Upgrade) ->
     catch
         ?WITH_STACKTRACE(C,T,S)
             ?DEBUG("~p ~p ~p", [C, T, S]),
-            ?WARN("Plugin ~p not available. It will not be used.", [Plugin]),
+            ?WARN("Errors loading plugin ~p. Run rebar3 with DEBUG=1 set to see errors.", [Plugin]),
             {[], State}
     end.
 
diff --git a/src/rebar_prv_shell.erl b/src/rebar_prv_shell.erl
index 434302b2..fbfc6a4f 100644
--- a/src/rebar_prv_shell.erl
+++ b/src/rebar_prv_shell.erl
@@ -104,6 +104,8 @@ do(Config) ->
     {ok, Config}.
 
 -spec format_error(any()) -> iolist().
+format_error({unknown_app, Unknown}) ->
+    io_lib:format("Applications list for shell contains an unrecognizable application definition: ~p", [Unknown]);
 format_error(Reason) ->
     io_lib:format("~p", [Reason]).
 
@@ -457,7 +459,9 @@ normalize_load_apps([{App, _Vsn, load} | T]) -> [App | normalize_load_apps(T)];
 normalize_load_apps([{_App, _Vsn, none} | T]) -> normalize_load_apps(T);
 normalize_load_apps([{App, _Vsn, Operator} | T]) when is_atom(Operator) ->
     [App | normalize_load_apps(T)];
-normalize_load_apps([App | T]) when is_atom(App) -> [App | normalize_load_apps(T)].
+normalize_load_apps([App | T]) when is_atom(App) -> [App | normalize_load_apps(T)];
+normalize_load_apps([Unknown | _]) ->
+    erlang:error(?PRV_ERROR({unknown_app, Unknown})).
 
 normalize_boot_apps([]) -> [];
 normalize_boot_apps([{_App, load} | T]) -> normalize_boot_apps(T);
@@ -467,7 +471,9 @@ normalize_boot_apps([{_App, _Vsn, none} | T]) -> normalize_boot_apps(T);
 normalize_boot_apps([{App, _Vsn, Operator} | T]) when is_atom(Operator) ->
     [App | normalize_boot_apps(T)];
 normalize_boot_apps([{App, _Vsn} | T]) -> [App | normalize_boot_apps(T)];
-normalize_boot_apps([App | T]) when is_atom(App) -> [App | normalize_boot_apps(T)].
+normalize_boot_apps([App | T]) when is_atom(App) -> [App | normalize_boot_apps(T)];
+normalize_boot_apps([Unknown | _]) ->
+    erlang:error(?PRV_ERROR({unknown_app, Unknown})).
 
 remove_error_handler(0) ->
     ?WARN("Unable to remove simple error_logger handler", []);