diff --git a/packages/fcl-image/src/fpreadtiff.pas b/packages/fcl-image/src/fpreadtiff.pas
index cfabb0ec41..5bcfe7dbe0 100644
--- a/packages/fcl-image/src/fpreadtiff.pas
+++ b/packages/fcl-image/src/fpreadtiff.pas
@@ -2098,7 +2098,7 @@ var
     // read two or three bytes
     if CurBitLength+SrcPosBit>16 then begin
       // read from three bytes
-      if SrcPos+3>Count then Error('LZW stream overrun');
+      if SrcPos+3>=Count then Error('LZW stream overrun');
       v:=PByte(Buffer)[SrcPos];
       inc(SrcPos);
       v:=(v shl 8)+PByte(Buffer)[SrcPos];
@@ -2107,7 +2107,7 @@ var
       v:=v shr (24-CurBitLength-SrcPosBit);
     end else begin
       // read from two bytes
-      if SrcPos+2>Count then Error('LZW stream overrun');
+      if SrcPos+2>=Count then Error('LZW stream overrun');
       v:=PByte(Buffer)[SrcPos];
       inc(SrcPos);
       v:=(v shl 8)+PByte(Buffer)[SrcPos];