issue in calling entry of matrix

I have a function that output is matrix as follows:

nth4[n_, m_] :=MatrixPower[DiagonalMatrix[ConstantArray[1, 2 n - 1], 1] + nth3[n],        m] 

Now when i try to call entery (1,2) using this way:

nth4[3, 2][[1, 2]] 

The result will be something like this:

{1, 1 + a, 1, 0, 0, 0} 

So basically return 6 entries from the first row instead one. why this happen. and is there is better way to call entry for this matrix?