An NMR spectrum of fumaric acid in acetone was run. The following results were obtained:

Question

1 sharp peak with 5 splittings at around 2 ppm (belonging to acetone)

1 broad peak at 3.789 ppm (integration 0.63)

1 sharp peak at 6.779 ppm (integration 1.00) (belonging to CH of fumaric acid)

1 even broader peak at 11.387 ppm (integration 0.52) (belonging to COOH of fumaric acid)

What is the cause of the peak at 3.789 ppm?