Mark object is disable for watches when debugging

Hi, 

According to the following documentation page

https://www.jetbrains.com/help/idea/setting-labels-to-variables-objects-and-watches.html

marking objects (put labels on them that can be used as variable names) should be possible. Nevertheless, I find it disabled.

To reproduce, just start a debugging session, set a breakpoint, in the variables pane click the + button, put the expression you want. Then, select the watch created, right click, mark object.

I am using

Community 2018.3.1, build #IC-183.4588.61, build on December 4, 2018

JRE: 1.8.0_152-release-1343-b16 amd64

JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o

Ubuntu 14.04

Thanks in advance