Pixel image reversal problem

I have copied an example of a mirror image generator from Delphi.com with a view to using it in a C++ builder project.

I get the following error in the iteration of the for loop:

First chance exception at $74CBC5AF. Exception class EInvalidOperation with message 'Can only modify an image if it contains a bitmap'. Process BitmapMirror.exe (7408)

The code is as follows:

void __fastcall TForm1::Button1Click(TObject *Sender)
{   // load an image file into Image1
	// reverse the horizontal image (make a mirror image) and display in Image2
	TBitmap *bm;
	int i,j;
	TColor c;

	if(OpenDialog1->Execute())
	{
		Image1->Picture->LoadFromFile(OpenDialog1->FileName);
	}
	bm = new TBitmap();
	bm->Height = Image1->Height;
	bm->Width = Image1->Width;
	for(i=0;i< Image1->Width;i++)
	for(j=0;j<Image1->Height;j++)
		bm->Canvas->Pixels[Image1->Width-i][j] = Image1->Canvas->Pixels[i][j];  // <<<<< This line produces the run-time error
	Image2->Picture->Assign(bm);
}

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