diff -up festival/speech_tools/siod/siod.cc.me festival/speech_tools/siod/siod.cc
--- festival/speech_tools/siod/siod.cc.me	2018-01-31 11:15:01.148463177 +0100
+++ festival/speech_tools/siod/siod.cc	2018-01-31 11:25:15.737355203 +0100
@@ -459,7 +459,7 @@ char **siod_variable_generator(char *tex
     matches = walloc(char *,siod_llength(lmatches)+1);
     for (l=lmatches,i=0; l; l=cdr(l),i++)
 	matches[i] = wstrdup(PNAME(car(l)));
-    matches[i] = '\0';
+    matches[i] = NULL;
 
     return matches;
 }
@@ -498,7 +498,7 @@ char **siod_command_generator (char *tex
     matches = walloc(char *,siod_llength(lmatches)+1);
     for (l=lmatches,i=0; l; l=cdr(l),i++)
 	matches[i] = wstrdup(PNAME(car(l)));
-    matches[i] = '\0';
+    matches[i] = NULL;
 
     return matches;
 }
