diff --git a/rtl/objpas/sysutils/sysformt.inc b/rtl/objpas/sysutils/sysformt.inc
index 165a4b0465..fd75384312 100644
--- a/rtl/objpas/sysutils/sysformt.inc
+++ b/rtl/objpas/sysutils/sysformt.inc
@@ -58,7 +58,10 @@ Var ChPos,OldPos,ArgPos,DoArg,Len : SizeInt;
     Procedure ReadIndex;
 
     begin
-      ReadInteger;
+      If Fmt[ChPos]<>':' then
+        ReadInteger
+      else
+        value:=0; // Delphi undocumented behaviour, assume 0, #11099
       If Fmt[ChPos]=':' then
         begin
         If Value=-1 then DoFormatError(feMissingArgument);