diff --git a/Lib/ctypes/test/test_arrays.py b/Lib/ctypes/test/test_arrays.py
index 6e562cfd24e663..9d50f18ec36937 100644
--- a/Lib/ctypes/test/test_arrays.py
+++ b/Lib/ctypes/test/test_arrays.py
@@ -183,6 +183,24 @@ class T(Array):
                 _type_ = c_int
                 _length_ = 1.87
 
+    def test_bad_length(self):
+        with self.assertRaises(ValueError):
+            class T(Array):
+                _type_ = c_int
+                _length_ = -1 << 1000
+        with self.assertRaises(ValueError):
+            class T(Array):
+                _type_ = c_int
+                _length_ = -1
+        with self.assertRaises(OverflowError):
+            class T(Array):
+                _type_ = c_int
+                _length_ = 1 << 1000
+        # _length_ might be zero.
+        class T(Array):
+            _type_ = c_int
+            _length_ = 0
+
     @unittest.skipUnless(sys.maxsize > 2**32, 'requires 64bit platform')
     @bigmemtest(size=_2G, memuse=1, dry_run=False)
     def test_large_array(self, size):
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-29-16-37-22.bpo-29843.RD5kyw.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-29-16-37-22.bpo-29843.RD5kyw.rst
new file mode 100644
index 00000000000000..741f493d97e7ce
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-29-16-37-22.bpo-29843.RD5kyw.rst	
@@ -0,0 +1,2 @@
+Raise `ValueError` instead of `OverflowError` in case of a negative
+``_length_`` in a `ctypes.Array` subclass. Patch by Oren Milman.
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index 3ae6348fef4317..4bef42e9438e4c 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -1391,6 +1391,7 @@ PyCArrayType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     StgDictObject *itemdict;
     PyObject *length_attr, *type_attr;
     Py_ssize_t length;
+    int overflow;
     Py_ssize_t itemsize, itemalign;
 
     /* create the new instance (which is a class,
@@ -1412,13 +1413,21 @@ PyCArrayType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         Py_XDECREF(length_attr);
         goto error;
     }
-    length = PyLong_AsSsize_t(length_attr);
+    length = PyLong_AsLongAndOverflow(length_attr, &overflow);
     Py_DECREF(length_attr);
-    if (length == -1 && PyErr_Occurred()) {
-        if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
+    /* PyLong_Check(length_attr) is true, so it is guaranteed that
+       no error occurred in PyLong_AsLongAndOverflow(). */
+    assert(!(length == -1 && PyErr_Occurred()));
+    if (overflow || length < 0) {
+        if (overflow > 0) {
             PyErr_SetString(PyExc_OverflowError,
                             "The '_length_' attribute is too large");
         }
+        else {
+            assert(overflow < 0 || length < 0);
+            PyErr_SetString(PyExc_ValueError,
+                            "The '_length_' attribute must be non-negative");
+        }
         goto error;
     }