diff --git a/src/cachetools/__init__.py b/src/cachetools/__init__.py
index 1b299c5..350122c 100644
--- a/src/cachetools/__init__.py
+++ b/src/cachetools/__init__.py
@@ -75,6 +75,8 @@ def __getitem__(self, key):
     def __setitem__(self, key, value):
         maxsize = self.__maxsize
         size = self.getsizeof(value)
+        if size < 0:
+            raise ValueError("value size must be non-negative")
         if size > maxsize:
             raise ValueError("value too large")
         if key not in self.__data or self.__size[key] < size:
diff --git a/tests/__init__.py b/tests/__init__.py
index 5856e71..a84f19e 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -284,6 +284,15 @@ def _test_getsizeof(self, cache):
     def test_getsizeof_param(self):
         self._test_getsizeof(self.Cache(maxsize=3, getsizeof=lambda x: x))
 
+    def test_getsizeof_negative(self):
+        cache = self.Cache(maxsize=3, getsizeof=lambda x: -1)
+
+        with self.assertRaises(ValueError):
+            cache[1] = 1
+
+        self.assertEqual(0, len(cache))
+        self.assertEqual(0, cache.currsize)
+
     def test_getsizeof_subclass(self):
         class Cache(self.Cache):
             @staticmethod