diff --git a/t/taplib.c b/t/taplib.c
index 256523f..5c23a6e 100644
--- a/t/taplib.c
+++ b/t/taplib.c
@@ -41,7 +41,7 @@ void is_int(int got, int expect, char *name)
   }
 }
 
-void is_str(char *got, char *expect, char *name)
+void is_str(const char *got, const char *expect, char *name)
 {
   if(strcmp(got, expect) == 0)
     ok(1, name);
diff --git a/t/taplib.h b/t/taplib.h
index b675329..45ce898 100644
--- a/t/taplib.h
+++ b/t/taplib.h
@@ -1,5 +1,5 @@
 void plan_tests(int n);
 void ok(int cmp, char *name);
 void is_int(int got, int expect, char *name);
-void is_str(char *got, char *expect, char *name);
+void is_str(const char *got, const char *expect, char *name);
 int exit_status(void);