diff --git a/rtl/objpas/sysconst.pp b/rtl/objpas/sysconst.pp
index 6aafbe4a68..eefa10b1a8 100644
--- a/rtl/objpas/sysconst.pp
+++ b/rtl/objpas/sysconst.pp
@@ -41,6 +41,7 @@ const
   SDispatchError         = 'No variant method call dispatch';
   SDivByZero             = 'Division by zero';
   SEndOfFile             = 'Read past end of file';
+  SErrPosToBigForLongint = 'File position %d too big to fit in 32-bit integer; Use Int64 overload instead';
   SErrInvalidDateMonthWeek = 'Year %d, month %d, Week %d and day %d is not a valid date.';
   SerrInvalidHourMinuteSecMsec = '%d:%d:%d.%d is not a valid time specification';
   SErrInvalidDateWeek    = '%d %d %d is not a valid dateweek';
diff --git a/rtl/unix/sysutils.pp b/rtl/unix/sysutils.pp
index 9077ee40a2..071f8b3419 100644
--- a/rtl/unix/sysutils.pp
+++ b/rtl/unix/sysutils.pp
@@ -525,8 +525,14 @@ end;
 
 Function FileSeek (Handle,FOffset,Origin : Longint) : Longint;
 
+Var
+  I : Int64;
+
 begin
-  result:=longint(FileSeek(Handle,int64(FOffset),Origin));
+  I:=FileSeek(Handle,int64(FOffset),Origin);
+  if I>High(Longint) then
+     Raise EInOutError.CreateFmt(SErrPosToBigForLongint,[I]);
+  result:=I;
 end;