diff --git a/Lib/ctypes/__init__.py b/Lib/ctypes/__init__.py
index 4ebdb797d7d..59373044956 100644
--- a/Lib/ctypes/__init__.py
+++ b/Lib/ctypes/__init__.py
@@ -478,7 +478,7 @@ def cast(obj, typ):
     return _cast(obj, obj, typ)
 
 _string_at = CFUNCTYPE(py_object, c_void_p, c_int)(_string_at_addr)
-def string_at(ptr, size=0):
+def string_at(ptr, size=-1):
     """string_at(addr[, size]) -> string
 
     Return the string at addr."""
@@ -490,7 +490,7 @@ except ImportError:
     pass
 else:
     _wstring_at = CFUNCTYPE(py_object, c_void_p, c_int)(_wstring_at_addr)
-    def wstring_at(ptr, size=0):
+    def wstring_at(ptr, size=-1):
         """wstring_at(addr[, size]) -> string
 
         Return the string at addr."""
diff --git a/Lib/ctypes/test/test_memfunctions.py b/Lib/ctypes/test/test_memfunctions.py
index fbae2ce1756..aef7a739fcf 100644
--- a/Lib/ctypes/test/test_memfunctions.py
+++ b/Lib/ctypes/test/test_memfunctions.py
@@ -14,6 +14,7 @@ class MemFunctionsTest(unittest.TestCase):
         self.failUnlessEqual(string_at(result), "Hello, World")
         self.failUnlessEqual(string_at(result, 5), "Hello")
         self.failUnlessEqual(string_at(result, 16), "Hello, World\0\0\0\0")
+        self.failUnlessEqual(string_at(result, 0), "")
 
     def test_memset(self):
         a = create_string_buffer(1000000)
@@ -54,6 +55,7 @@ class MemFunctionsTest(unittest.TestCase):
             self.failUnlessEqual(wstring_at(a), "Hello, World")
             self.failUnlessEqual(wstring_at(a, 5), "Hello")
             self.failUnlessEqual(wstring_at(a, 16), "Hello, World\0\0\0\0")
+            self.failUnlessEqual(wstring_at(a, 0), "")
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS b/Misc/NEWS
index 525c9bd9f08..a5b3fc7998e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -197,6 +197,9 @@ Extension Modules
 Library
 -------
 
+- Fix bug #1646630: ctypes.string_at(buf, 0) and ctypes.wstring_at(buf, 0)
+  returned string up to the first NUL character.
+
 - Bug #1637850:  make_table in difflib did not work with unicode
 
 - Bugs #1676321:  the empty() function in sched.py returned the wrong result
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index 89c5aaea55b..69d3157bcfa 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -4531,9 +4531,9 @@ create_comerror(void)
 #endif
 
 static PyObject *
-string_at(const char *ptr, Py_ssize_t size)
+string_at(const char *ptr, int size)
 {
-	if (size == 0)
+	if (size == -1)
 		return PyString_FromString(ptr);
 	return PyString_FromStringAndSize(ptr, size);
 }
@@ -4618,7 +4618,7 @@ cast(void *ptr, PyObject *src, PyObject *ctype)
 static PyObject *
 wstring_at(const wchar_t *ptr, int size)
 {
-	if (size == 0)
+	if (size == -1)
 		size = wcslen(ptr);
 	return PyUnicode_FromWideChar(ptr, size);
 }