Fix

src/lager_logger_formatter.erl

@@ -88,7 +88,9 @@ format(#{msg := {report, _Report}, meta := Metadata} = Event, #{report_cb := Fun
 format(#{level := _Level, msg := {report, Report}, meta := #{report_cb := Fun}} = Event, Config) when is_function(Fun, 1) ->
     case Fun(Report) of
         {Format, Args} when is_list(Format), is_list(Args) ->
-            format(Event#{msg => {Format, Args}}, Config)
+            format(Event#{msg => {Format, Args}}, Config);
+        String ->
+            format(Event#{msg => String}, Config)
     end;
 format(#{level := Level, msg := {string, String}, meta := Metadata}, Config) ->
     do_format(Level, String, Metadata, Config);