Set fig additional caption really optional

config/thesis_commands.tex

@@ -32,7 +32,7 @@
 % Images
 % example: \fig[1]{universe}{The Universe}[Additional caption text, not shown in the list of figures]
 % reference later with: \figref{universe}
-\RenewDocumentCommand{\fig}{O{1} O{htbp} m m o}{% #1 = width, #2 = position, #3 = label, #4 = caption, #5 = additional caption
+\RenewDocumentCommand{\fig}{O{1} O{htbp} m m O{}}{% #1 = width, #2 = position, #3 = label, #4 = caption, #5 = additional caption
     \begin{figure}[#2]
         \centering%
         \includegraphics[width=#1\linewidth]{#3}%