diff --git a/doc/portability.rst b/doc/portability.rst
index 0a49e2e..dc6c5eb 100644
--- a/doc/portability.rst
+++ b/doc/portability.rst
@@ -20,6 +20,16 @@ such values, as containers manage the reference count of their
 contained values. Bugs involving concurrent incrementing or
 decrementing of deference counts may be hard to track.
 
+The encoding functions (:func:`json_dumps()` and friends) track
+reference loops by modifying the internal state of objects and arrays.
+For this reason, encoding functions must not be run on the same JSON
+values in two separate threads at the same time. As already noted
+above, be especially careful if two arrays or objects share their
+contained values with another array or object.
+
+If you want to make sure that two JSON value hierarchies do not
+contain shared values, use :func:`json_deep_copy()` to make copies.
+
 Locale
 ------
 
diff --git a/src/dump.c b/src/dump.c
index 33112ba..089474d 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -159,8 +159,10 @@ static int object_key_compare_keys(const void *key1, const void *key2)
 
 static int object_key_compare_serials(const void *key1, const void *key2)
 {
-    return (*(const object_key_t **)key1)->serial -
-           (*(const object_key_t **)key2)->serial;
+    size_t a = (*(const object_key_t **)key1)->serial;
+    size_t b = (*(const object_key_t **)key2)->serial;
+
+    return a < b ? -1 : a == b ? 0 : 1;
 }
 
 static int do_dump(const json_t *json, size_t flags, int depth,
diff --git a/src/load.c b/src/load.c
index ef65f0f..f5cbf68 100644
--- a/src/load.c
+++ b/src/load.c
@@ -869,13 +869,19 @@ json_t *json_loads(const char *string, size_t flags, json_error_t *error)
     json_t *result;
     string_data_t stream_data;
 
+    jsonp_error_init(error, "<string>");
+
+    if (string == NULL) {
+        error_set(error, NULL, "wrong arguments");
+        return NULL;
+    }
+
     stream_data.data = string;
     stream_data.pos = 0;
 
     if(lex_init(&lex, string_get, (void *)&stream_data))
         return NULL;
 
-    jsonp_error_init(error, "<string>");
     result = parse_json(&lex, flags, error);
 
     lex_close(&lex);
@@ -907,6 +913,13 @@ json_t *json_loadb(const char *buffer, size_t buflen, size_t flags, json_error_t
     json_t *result;
     buffer_data_t stream_data;
 
+    jsonp_error_init(error, "<buffer>");
+
+    if (buffer == NULL) {
+        error_set(error, NULL, "wrong arguments");
+        return NULL;
+    }
+
     stream_data.data = buffer;
     stream_data.pos = 0;
     stream_data.len = buflen;
@@ -914,7 +927,6 @@ json_t *json_loadb(const char *buffer, size_t buflen, size_t flags, json_error_t
     if(lex_init(&lex, buffer_get, (void *)&stream_data))
         return NULL;
 
-    jsonp_error_init(error, "<buffer>");
     result = parse_json(&lex, flags, error);
 
     lex_close(&lex);
@@ -927,15 +939,21 @@ json_t *json_loadf(FILE *input, size_t flags, json_error_t *error)
     const char *source;
     json_t *result;
 
-    if(lex_init(&lex, (get_func)fgetc, input))
-        return NULL;
-
     if(input == stdin)
         source = "<stdin>";
     else
         source = "<stream>";
 
     jsonp_error_init(error, source);
+
+    if (input == NULL) {
+        error_set(error, NULL, "wrong arguments");
+        return NULL;
+    }
+
+    if(lex_init(&lex, (get_func)fgetc, input))
+        return NULL;
+
     result = parse_json(&lex, flags, error);
 
     lex_close(&lex);
@@ -949,6 +967,11 @@ json_t *json_load_file(const char *path, size_t flags, json_error_t *error)
 
     jsonp_error_init(error, path);
 
+    if (path == NULL) {
+        error_set(error, NULL, "wrong arguments");
+        return NULL;
+    }
+
     fp = fopen(path, "rb");
     if(!fp)
     {
diff --git a/src/value.c b/src/value.c
index d0517d5..5ef4138 100644
--- a/src/value.c
+++ b/src/value.c
@@ -186,7 +186,9 @@ int json_object_clear(json_t *json)
         return -1;
 
     object = json_to_object(json);
+
     hashtable_clear(&object->hashtable);
+    object->serial = 0;
 
     return 0;
 }
diff --git a/test/suites/api/test_load.c b/test/suites/api/test_load.c
index fb4c734..8143d46 100644
--- a/test/suites/api/test_load.c
+++ b/test/suites/api/test_load.c
@@ -76,10 +76,33 @@ static void decode_any()
     json_decref(json);
 }
 
+static void load_wrong_args()
+{
+    json_t *json;
+    json_error_t error;
+
+    json = json_loads(NULL, 0, &error);
+    if (json)
+        fail("json_loads should return NULL if the first argument is NULL");
+
+    json = json_loadb(NULL, 0, 0, &error);
+    if (json)
+        fail("json_loadb should return NULL if the first argument is NULL");
+
+    json = json_loadf(NULL, 0, &error);
+    if (json)
+        fail("json_loadf should return NULL if the first argument is NULL");
+
+    json = json_load_file(NULL, 0, &error);
+    if (json)
+        fail("json_loadf should return NULL if the first argument is NULL");
+}
+
 static void run_tests()
 {
     file_not_found();
     reject_duplicates();
     disable_eof_check();
     decode_any();
+    load_wrong_args();
 }