diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 8b4d2689930..c41722c72f0 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -717,6 +717,12 @@ class ReTests(unittest.TestCase):
         self.assertRaises(ValueError, re.compile, '(?a)\w', re.UNICODE)
         self.assertRaises(ValueError, re.compile, '(?au)\w')
 
+    def test_dealloc(self):
+        # issue 3299: check for segfault in debug build
+        import _sre
+        long_overflow = sys.maxsize + 2
+        self.assertRaises(TypeError, re.finditer, "a", {})
+        self.assertRaises(OverflowError, _sre.compile, "abc", 0, [long_overflow])
 
 def run_re_tests():
     from test.re_tests import benchmarks, tests, SUCCEED, FAIL, SYNTAX_ERROR
diff --git a/Misc/NEWS b/Misc/NEWS
index a577e65a239..f2e2fed6ca6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -213,6 +213,9 @@ C-API
 Library
 -------
 
+- Issue #3299: Fix possible crash in the _sre module when given bad
+  argument values in debug mode.  Patch by Victor Stinner.
+
 - Issue #2846: Add support for gzip.GzipFile reading zero-padded files.
   Patch by Brian Curtin.
 
diff --git a/Modules/_sre.c b/Modules/_sre.c
index 596fd19dfdb..7b3ee5fe3a8 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -2674,6 +2674,10 @@ _compile(PyObject* self_, PyObject* args)
     self = PyObject_NEW_VAR(PatternObject, &Pattern_Type, n);
     if (!self)
         return NULL;
+    self->weakreflist = NULL;
+    self->pattern = NULL;
+    self->groupindex = NULL;
+    self->indexgroup = NULL;
 
     self->codesize = n;
 
@@ -2689,7 +2693,7 @@ _compile(PyObject* self_, PyObject* args)
     }
 
     if (PyErr_Occurred()) {
-        PyObject_DEL(self);
+        Py_DECREF(self);
         return NULL;
     }
 
@@ -3730,7 +3734,7 @@ static void
 scanner_dealloc(ScannerObject* self)
 {
     state_fini(&self->state);
-    Py_DECREF(self->pattern);
+    Py_XDECREF(self->pattern);
     PyObject_DEL(self);
 }
 
@@ -3860,10 +3864,11 @@ pattern_scanner(PatternObject* pattern, PyObject* args)
     self = PyObject_NEW(ScannerObject, &Scanner_Type);
     if (!self)
         return NULL;
+    self->pattern = NULL;
 
     string = state_init(&self->state, pattern, string, start, end);
     if (!string) {
-        PyObject_DEL(self);
+        Py_DECREF(self);
         return NULL;
     }