Trouble with .css.map tech

[qfox]

$ enb -V
0.16.0

13:52:02.982 - [error] [/.../_20-popup.css.map] - 5350ms
Error: There is no tech for target .../_20-popup.css.map.
    at Error (native)
    at TargetNotFoundError (.../enb/lib/errors/target-not-found-error.js:11:23)
    at .../enb/lib/node/node.js:487:27
    at Array.map (native)
    at .../enb/lib/node/node.js:483:36
    at Array.<anonymous> (.../vow/lib/vow.js:711:56)
    at Immediate.callFns [as _onImmediate] (.../vow/lib/vow.js:23:35)
    at processImmediate [as _immediateCallback] (timers.js:383:17)