Photoshop CS6 - Why is there a no entry sign over the curves 'hand' adjustment tool, the one where you slide it up or down etcWhat am I doing wrong?

IN the curves panel the cursor turn into a 'no entry' sign when I hover over the 'hand' tool thing I normally click on to make adjustments to the curves, I have never seen this before and don't know why its there, but the pencil looking tool is available

what's going on here - I am doing somthing wrong no doubt?