1. A method in a decoder for decoding a bit stream representing a residual image block to be added to a predicted image block for obtaining a reconstructed image block, the method comprising: receiving the bit stream, entropy decoding the received bit stream to identify transform coefficients of different frequencies of a transform block, determining if the transform coefficients comprise a single non-zero coefficient of a lowest frequency that is most sensitive to changes of an average value of the residual image block, if the transform coefficients comprise said single non-zero coefficient, generating a residual block with a level depending on a value of the single non-zero coefficient, and otherwise if the transform coefficients do not comprise said single non-zero coefficient, generating a residual block by deploying an inverse of the spatial 2 dimension (2D) transform.