A question about Maxwell's Equations

These notes on Electromagnetism (chapter 3.1.2, section "A Solenoid") say the following:

We solve Ampère’s law in differential form. Anywhere other than
the surface of the solenoid, we have $mathbf{J}=0$ and
$$nablatimesmathbf{B}=0quadRightarrowquadfrac{dB}{dr}=0quadRightarrow quad B(r)=text{constant}$$

Why does $nablatimes B=0$ imply that $frac{dB}{dr}=0$?