wrong-type-argument stringp nil

[Colin Baxter <m43cap@yandex.com>]

Hello,

I getting the Lisp error: (wrong-type-argument stringp nil) when I 'C-c
C-c' the babel gnuplot source code "poynting", even though the pdf graph
is produced correctly. Any help at debugging would be much appreciated.

#+name: poynting
|  i |      R |    P/M |
|  0 | 0.0400 | 0.4000 |
| 20 | 0.0402 | 0.4240 |
| 40 | 0.0458 | 0.4925 |
| 50 | 0.0572 | 0.5310 |
| 60 | 0.0893 | 0.5720 |
| 65 | 0.1205 | 0.5771 |
| 70 | 0.1710 | 0.5683 |
| 75 | 0.2531 | 0.5329 |
| 80 | 0.3878 | 0.4521 |
| 89 | 0.9044 | 0.0738 |
| 90 | 1.0000 | 0.0000 |
#+begin_src gnuplot :var data=poynting :file poynting.pdf :results file
  set title 'Plot of Data from Table 1'
  show title
  set key at 30,0.8
  unset key
  set xlabel 'Angle'
  set ylabel ''
  set style datapoints
  set style line 1 lc rgb '#0060ad'  # --- blue
  set style line 2 lc rgb '#dd181f'  # --- red
  set label 'P/M' at 10,0.3700
  set label 'R' at 35,0.0855
  plot data using 1:2 with points ls 1, \
  data using 1:2 smooth csplines with lines, \
  data using 1:3 with points ls 2, \
  data using 1:3 smooth csplines with lines
#+end_src


Best wishes,

Colin Baxter.