diff --git a/tools/mksyscall.c b/tools/mksyscall.c
index 953c132f2e1..91feb4a3894 100644
--- a/tools/mksyscall.c
+++ b/tools/mksyscall.c
@@ -142,8 +142,8 @@ static void get_actualparmtype(const char *arg, char *actual)
 
 static void get_fieldname(const char *arg, char *fieldname)
 {
-  char *pactual = strchr(arg, '|');
-  char *pstart;
+  const char *pactual = strchr(arg, '|');
+  const char *pstart;
 
   if (pactual)
     {