gdb: Meaning of tstart error “You can't do that when your target is `exec'”

I would like to record the value of a local variable, t, each time that the program reaches a certain line. Accordingly, I tried:

(gdb) trace stoer_wagner_min_cut.hpp :197
Tracepoint 1 at 0x4123a0: file ./boost/graph/stoer_wagner_min_cut.hpp, line 197.
(gdb) actions
Enter actions for tracepoint 1, one per line.
End with a line saying just "end".
> collect t
> end
(gdb) tstart
You can't do that when your target is `exec'
(gdb) break main
Breakpoint 2 at 0x401448: file time_stoer_wagner.cpp, line 50.
(gdb) run
Starting program: C:\Users\Daniel\Documents\projects\stoer_wagner_min_cut/time_stoer_wagner.exe
[New Thread 3908.0x39c]

Breakpoint 2, main () at time_stoer_wagner.cpp:50
50        std::ifstream ifs("prgen_500_50_2.txt");
(gdb) tstart
You can't do that when your target is `child'

but the error messages "You can't do that when your target is `exec'" and "You can't do that when your target is `child'" are not helpful to me. What do these errors mean?