[Lazarus] Why is setlength not allowed?

[Bo Berglund]

I am trying to write a simple I2C interface program for Raspberry Pi
but I am getting an error I don't understand...

function TRaspiI2C.ReadI2CBytes(addr: integer; count: integer; buf:
array of byte): integer;
var
  i: integer;
begin
  try
    if count <> length(buf) then
      SetLength (buf, count);  <== ERROR HERE!
    for i := 0 to count -1 do
      buf[i] := ReadI2CByte(addr + i);
    Result := count;
  except
    Result := 0;
  end;
end;

The error message states:

rpii2ccomm.pas(158,7) Error: Type mismatch

I have used setlength on dynamic byte arrays many times before and
never gotten this strange error message.
What am I doing wrong???

I have tried declaring buf as var in the call but it does not help.

(addr: integer; count: integer; var buf: array of byte): integer;

The same error happens.

Must be something basic I have missed.

Lazarus placed this at the top of the unit when I created it, if that
matters:

{$mode objfpc}{$H+} 

Using FPC 3.0.4 and Lazarus 2.0.6 on Raspbian Buster


-- 
Bo Berglund
Developer in Sweden