diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
index aa36b09..9f79b02 100644
--- a/Lib/test/test_StringIO.py
+++ b/Lib/test/test_StringIO.py
@@ -62,6 +62,7 @@
         eq(f.getvalue(), 'abcde')
         f.write('xyz')
         eq(f.getvalue(), 'abcdexyz')
+        self.assertRaises(IOError, f.truncate, -1)
         f.close()
         self.assertRaises(ValueError, f.write, 'frobnitz')
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 6fd73f6..9b2aeba 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -101,6 +101,9 @@
 Library
 -------
 
+- cStringIO.truncate(-1) now raises an IOError, like StringIO and
+  regular files.
+
 - Patch #1472877: Fix Tix subwidget name resolution.
 
 - Patch #1594554: Always close a tkSimpleDialog on ok(), even
diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index 100891b..3f762b0 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -289,7 +289,17 @@
 	
         if (!IO__opencheck(self)) return NULL;
         if (!PyArg_ParseTuple(args, "|n:truncate", &pos)) return NULL;
-        if (pos < 0) pos = self->pos;
+
+	if (PyTuple_Size(args) == 0) {
+		/* No argument passed, truncate to current position */
+		pos = self->pos;
+	}
+
+        if (pos < 0) {
+		errno = EINVAL;
+		PyErr_SetFromErrno(PyExc_IOError);
+		return NULL;
+	}
 
         if (self->string_size > pos) self->string_size = pos;
         self->pos = self->string_size;