diff --git a/Lib/StringIO.py b/Lib/StringIO.py
index 232009f..300dd5a 100644
--- a/Lib/StringIO.py
+++ b/Lib/StringIO.py
@@ -158,7 +158,7 @@
             newpos = self.len
         else:
             newpos = i+1
-        if length is not None:
+        if length is not None and length > 0:
             if self.pos + length < newpos:
                 newpos = self.pos + length
         r = self.buf[self.pos:newpos]
diff --git a/Lib/test/test_StringIO.py b/Lib/test/test_StringIO.py
index 58cbec0..81f81e5 100644
--- a/Lib/test/test_StringIO.py
+++ b/Lib/test/test_StringIO.py
@@ -28,6 +28,8 @@
         eq(self._fp.read(10), self._line[:10])
         eq(self._fp.readline(), self._line[10:] + '\n')
         eq(len(self._fp.readlines(60)), 2)
+        self._fp.seek(0)
+        eq(self._fp.readline(-1), self._line + '\n')
 
     def test_writes(self):
         f = self.MODULE.StringIO()
diff --git a/Misc/NEWS b/Misc/NEWS
index d9cb038..a933f62 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -35,6 +35,9 @@
 Library
 -------
 
+- Issue #7348: StringIO.StringIO.readline(-1) now acts as if it got no argument
+  like other file objects.
+
 - Issue #5949: fixed IMAP4_SSL hang when the IMAP server response is
   missing proper end-of-line termination.