diff --git a/testcrypt.c b/testcrypt.c
index 06f2f32b..0035c0c0 100644
--- a/testcrypt.c
+++ b/testcrypt.c
@@ -495,14 +495,19 @@ static void return_boolean(strbuf *out, bool b)
     strbuf_catf(out, "%s\n", b ? "true" : "false");
 }
 
-static void return_val_string_asciz(strbuf *out, char *s)
+static void return_val_string_asciz_const(strbuf *out, const char *s)
 {
     strbuf *sb = strbuf_new();
     put_data(sb, s, strlen(s));
-    sfree(s);
     return_val_string(out, sb);
 }
 
+static void return_val_string_asciz(strbuf *out, char *s)
+{
+    return_val_string_asciz_const(out, s);
+    sfree(s);
+}
+
 #define NULLABLE_RETURN_WRAPPER(type_name, c_type)                      \
     static void return_opt_##type_name(strbuf *out, c_type ptr)         \
     {                                                                   \