Invalid Variant Type when initializing Variant

I am using D7.1.   I am working with some maintenance code that randomly
gives an "Invalid Variant Type" exception (EVariantBadVarTypeError).  I have
tracked it down the following code.

TSearchType = record
    Cache : Boolean;
    Value : Variant;
    Key   : Longint;
    Tree : TSortNode;
end;

TSearchTypeArray = array [0..100000] of TSearchType;
PSearchTypeArray = ^TSearchTypeArray;

TMyClass = class
  private
    SearchTable : PSearchTypeArray;
    NumItems  : Longint;
    MaxedLimit : Boolean;
  public
    constructor Create(size : Longint);
end

constructor TMyClass.Create(size : Longint);
var
    i : Longint;
begin
    inherited Create;

    // Allocate the Search table entries.
    NumItems := size;
    GetMem(SearchTable, NumItems * SizeOf(TSearchType));

    // Initialize the Search table entries to Unassigned.
    for i := 0 to NumItems - 1 do
    begin
      SearchTable^[i].Value := Unassigned; // error randomly occurs on this
line
      SearchTable^[i].Cache:=false;
      SearchTable^[i].tree:=NIL;
    end;
end;

I was under the impression that I should only get this error if I try to
assign a normal datatype to an unassigned variant.

Any ideas?