diff --git a/calibrate_ppa.c b/calibrate_ppa.c
index d2eddb7..59f6231 100644
--- a/calibrate_ppa.c
+++ b/calibrate_ppa.c
@@ -81,7 +81,7 @@
   FILE *outFile = NULL ;
   int testPattern = 0;
   char *out_filepath = NULL;
-  char c;
+  int c;
   static struct option long_opts[] = {
     {"gamma",0,0,'g'},
     {"align",0,0,0},