error 'Value of 'NIL in ... ' in make-graph

When I run just the simple example on the readme page, I get an error and backtrace like this:
```
Value of 'NIL in
(THE (OR STRING SB-DI::COMPILED-DEBUG-FUN) (PROGN 'NIL))
is
NIL,
not a
(OR (VECTOR CHARACTER) BASE-STRING SB-DI::COMPILED-DEBUG-FUN).
[Condition of type SIMPLE-TYPE-ERROR]
...
0: (SB-C::%COMPILE-TIME-TYPE-ERROR (NIL) (OR (VECTOR CHARACTER) BASE-STRING SB-DI::COMPILED-DEBUG-FUN) # ((QUOTE NIL)) "(THE (OR STRING SB-DI::COMPILED-DEBUG-FUN) (PROGN 'NIL))" NIL)
1: ((FLET SB-PCL::INITIALIZE-IT) # #)
2: ((LAMBDA ()))
3: (FLAMEGRAPH::MAKE-GRAPH)
...
```
Environment: SBCL 2.0.11 on MacOS

To reproduce:
```
(ql:quickload :flamegraph)
(defun foo () (sleep 0.01))
(defun bar () (sleep 0.05))
(defun blah () (loop repeat 1000 do (foo) (bar)))
(flamegraph:save-flame-graph ("/tmp/foo.stack") (blah))
```