Although "Warum betrübst du dich, mein Herz" (What is it troubles thee, my heart) BWV 138 is based on the chorale of the same name, it is nevertheless not regarded as one of Bach's chorale cantatas. in this unusual composition from Bach's first cycle of cantatas, further chorale verses were not paraphrased in the freely-composed sections (as in the chorale cantatas). Instead the chorale occurs in dialog with the recitatives. It assumes the role of the comforter, whilst doubts are expressed through the recitatives. This leads to recitative and chorale being merged into larger movements. in the only aria in the cantata, trust in God finally prevails. The cantata ends with a final chorale verse; in the interludes between the lines, Bach ignites a roaring virtuoso firework display.