diff -r 03ff92b26fa9 Lib/test/test_capi.py
--- a/Lib/test/test_capi.py	Mon Apr 13 17:48:40 2015 -0400
+++ b/Lib/test/test_capi.py	Mon Apr 13 17:18:05 2015 -0500
@@ -123,7 +123,7 @@
         self.assertEqual(_testcapi.no_docstring.__doc__, None)
         self.assertEqual(_testcapi.no_docstring.__text_signature__, None)
 
-        self.assertEqual(_testcapi.docstring_empty.__doc__, "")
+        self.assertEqual(_testcapi.docstring_empty.__doc__, None)
         self.assertEqual(_testcapi.docstring_empty.__text_signature__, None)
 
         self.assertEqual(_testcapi.docstring_no_signature.__doc__,
@@ -150,6 +150,10 @@
             "This docstring has a valid signature.")
         self.assertEqual(_testcapi.docstring_with_signature.__text_signature__, "($module, /, sig)")
 
+        self.assertEqual(_testcapi.docstring_with_signature_but_no_doc.__doc__, None)
+        self.assertEqual(_testcapi.docstring_with_signature_but_no_doc.__text_signature__,
+            "($module, /, sig)")
+
         self.assertEqual(_testcapi.docstring_with_signature_and_extra_newlines.__doc__,
             "\nThis docstring has a valid signature and some extra newlines.")
         self.assertEqual(_testcapi.docstring_with_signature_and_extra_newlines.__text_signature__,
diff -r 03ff92b26fa9 Lib/test/test_inspect.py
--- a/Lib/test/test_inspect.py	Mon Apr 13 17:48:40 2015 -0400
+++ b/Lib/test/test_inspect.py	Mon Apr 13 17:18:05 2015 -0500
@@ -1864,6 +1864,9 @@
         test_unbound_method(dict.__delitem__)
         test_unbound_method(property.__delete__)
 
+        # Regression test for issue #20586
+        test_callable(_testcapi.docstring_with_signature_but_no_doc)
+
     @cpython_only
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")
diff -r 03ff92b26fa9 Modules/_testcapimodule.c
--- a/Modules/_testcapimodule.c	Mon Apr 13 17:48:40 2015 -0400
+++ b/Modules/_testcapimodule.c	Mon Apr 13 17:18:05 2015 -0500
@@ -3083,6 +3083,12 @@
 "This docstring has a valid signature."
 );
 
+PyDoc_STRVAR(docstring_with_signature_but_no_doc,
+"docstring_with_signature_but_no_doc($module, /, sig)\n"
+"--\n"
+"\n"
+);
+
 PyDoc_STRVAR(docstring_with_signature_and_extra_newlines,
 "docstring_with_signature_and_extra_newlines($module, /, parameter)\n"
 "--\n"
@@ -3635,6 +3641,9 @@
     {"docstring_with_signature",
         (PyCFunction)test_with_docstring, METH_NOARGS,
         docstring_with_signature},
+    {"docstring_with_signature_but_no_doc",
+        (PyCFunction)test_with_docstring, METH_NOARGS,
+        docstring_with_signature_but_no_doc},
     {"docstring_with_signature_and_extra_newlines",
         (PyCFunction)test_with_docstring, METH_NOARGS,
         docstring_with_signature_and_extra_newlines},
diff -r 03ff92b26fa9 Objects/typeobject.c
--- a/Objects/typeobject.c	Mon Apr 13 17:48:40 2015 -0400
+++ b/Objects/typeobject.c	Mon Apr 13 17:18:05 2015 -0500
@@ -137,7 +137,7 @@
 {
     const char *doc = _PyType_DocWithoutSignature(name, internal_doc);
 
-    if (!doc) {
+    if (!doc || *doc == '\0') {
         Py_INCREF(Py_None);
         return Py_None;
     }
diff -r 03ff92b26fa9 Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py	Mon Apr 13 17:48:40 2015 -0400
+++ b/Tools/clinic/clinic.py	Mon Apr 13 17:18:05 2015 -0500
@@ -66,6 +66,8 @@
 
 unknown = Unknown()
 
+sig_end_marker = '--'
+
 
 _text_accumulator_nt = collections.namedtuple("_text_accumulator", "text append output")
 
@@ -559,8 +561,13 @@
             add(quoted_for_c_string(line))
             add('\\n"\n')
 
-        text.pop()
-        add('"')
+        if text[-2] == sig_end_marker:
+            # If we only have a signature, add the blank line that the
+            # __text_signature__ getter expects to be there.
+            add('"\\n"')
+        else:
+            text.pop()
+            add('"')
         return ''.join(text)
 
     def output_templates(self, f):
@@ -4015,7 +4022,7 @@
         #     add(f.return_converter.py_default)
 
         if not f.docstring_only:
-            add("\n--\n")
+            add("\n" + sig_end_marker + "\n")
 
         docstring_first_line = output()