diff --git a/misc/json.c b/misc/json.c
index 608cfad861..d30450c49d 100644
--- a/misc/json.c
+++ b/misc/json.c
@@ -255,6 +255,8 @@ static const char special_escape[] = {
 
 static void write_json_str(bstr *b, unsigned char *str)
 {
+    assert(str);
+
     APPEND(b, "\"");
     while (1) {
         unsigned char *cur = str;
diff --git a/player/command.c b/player/command.c
index 4ac0a4556a..64d1b4d310 100644
--- a/player/command.c
+++ b/player/command.c
@@ -3605,7 +3605,8 @@ static int mp_property_option_info(void *ctx, struct m_property *prop,
 
         union m_option_value def = m_option_value_default;
         const void *def_ptr = m_config_get_co_default(mpctx->mconfig, co);
-        if (def_ptr && opt->type->size > 0)
+        bool has_default = def_ptr && opt->type->size > 0;
+        if (has_default)
             memcpy(&def, def_ptr, opt->type->size);
 
         bool has_minmax = opt->min < opt->max &&
@@ -3643,7 +3644,7 @@ static int mp_property_option_info(void *ctx, struct m_property *prop,
             {"set-from-commandline",    SUB_PROP_BOOL(co->is_set_from_cmdline)},
             {"set-locally",             SUB_PROP_BOOL(co->is_set_locally)},
             {"expects-file",            SUB_PROP_BOOL(opt->flags & M_OPT_FILE)},
-            {"default-value",           *opt, def},
+            {"default-value",           *opt, def, .unavailable = !has_default},
             {"min",                     SUB_PROP_DOUBLE(opt->min),
              .unavailable = !(has_minmax && opt->min != DBL_MIN)},
             {"max",                     SUB_PROP_DOUBLE(opt->max),