diff --git a/Modules/_json.c b/Modules/_json.c
index 95c658c..40c2ced 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -246,6 +246,7 @@
         }
     }
     output[chars++] = '"';
+    assert(_PyUnicode_CheckConsistency(rval, 1));
     return rval;
 }
 
diff --git a/Modules/md5module.c b/Modules/md5module.c
index 86f602e..ee44c48 100644
--- a/Modules/md5module.c
+++ b/Modules/md5module.c
@@ -397,6 +397,7 @@
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 
diff --git a/Modules/sha1module.c b/Modules/sha1module.c
index 30e5c50..daea887 100644
--- a/Modules/sha1module.c
+++ b/Modules/sha1module.c
@@ -373,6 +373,7 @@
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 
diff --git a/Modules/sha256module.c b/Modules/sha256module.c
index f1ef329..76d91af 100644
--- a/Modules/sha256module.c
+++ b/Modules/sha256module.c
@@ -466,6 +466,7 @@
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 
diff --git a/Modules/sha512module.c b/Modules/sha512module.c
index 4f5a113..88f8a64 100644
--- a/Modules/sha512module.c
+++ b/Modules/sha512module.c
@@ -532,6 +532,7 @@
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index 2e6be43..b07be26 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -626,6 +626,7 @@
             *p++ = c;
     }
     *p++ = quote;
+    assert(_PyUnicode_CheckConsistency(v, 1));
     return v;
 }
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 364de90..60b0a1f 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -967,7 +967,7 @@
     PyObject *obj;
     PyCompactUnicodeObject *unicode;
     void *data;
-    int kind_state;
+    enum PyUnicode_Kind kind;
     int is_sharing, is_ascii;
     Py_ssize_t char_size;
     Py_ssize_t struct_size;
@@ -986,17 +986,17 @@
     is_sharing = 0;
     struct_size = sizeof(PyCompactUnicodeObject);
     if (maxchar < 128) {
-        kind_state = PyUnicode_1BYTE_KIND;
+        kind = PyUnicode_1BYTE_KIND;
         char_size = 1;
         is_ascii = 1;
         struct_size = sizeof(PyASCIIObject);
     }
     else if (maxchar < 256) {
-        kind_state = PyUnicode_1BYTE_KIND;
+        kind = PyUnicode_1BYTE_KIND;
         char_size = 1;
     }
     else if (maxchar < 65536) {
-        kind_state = PyUnicode_2BYTE_KIND;
+        kind = PyUnicode_2BYTE_KIND;
         char_size = 2;
         if (sizeof(wchar_t) == 2)
             is_sharing = 1;
@@ -1007,7 +1007,7 @@
                             "invalid maximum character passed to PyUnicode_New");
             return NULL;
         }
-        kind_state = PyUnicode_4BYTE_KIND;
+        kind = PyUnicode_4BYTE_KIND;
         char_size = 4;
         if (sizeof(wchar_t) == 4)
             is_sharing = 1;
@@ -1041,7 +1041,7 @@
     _PyUnicode_LENGTH(unicode) = size;
     _PyUnicode_HASH(unicode) = -1;
     _PyUnicode_STATE(unicode).interned = 0;
-    _PyUnicode_STATE(unicode).kind = kind_state;
+    _PyUnicode_STATE(unicode).kind = kind;
     _PyUnicode_STATE(unicode).compact = 1;
     _PyUnicode_STATE(unicode).ready = 1;
     _PyUnicode_STATE(unicode).ascii = is_ascii;
@@ -1049,19 +1049,19 @@
         ((char*)data)[size] = 0;
         _PyUnicode_WSTR(unicode) = NULL;
     }
-    else if (kind_state == PyUnicode_1BYTE_KIND) {
+    else if (kind == PyUnicode_1BYTE_KIND) {
         ((char*)data)[size] = 0;
         _PyUnicode_WSTR(unicode) = NULL;
         _PyUnicode_WSTR_LENGTH(unicode) = 0;
         unicode->utf8 = NULL;
         unicode->utf8_length = 0;
-        }
+    }
     else {
         unicode->utf8 = NULL;
         unicode->utf8_length = 0;
-        if (kind_state == PyUnicode_2BYTE_KIND)
+        if (kind == PyUnicode_2BYTE_KIND)
             ((Py_UCS2*)data)[size] = 0;
-        else /* kind_state == PyUnicode_4BYTE_KIND */
+        else /* kind == PyUnicode_4BYTE_KIND */
             ((Py_UCS4*)data)[size] = 0;
         if (is_sharing) {
             _PyUnicode_WSTR_LENGTH(unicode) = size;
@@ -1072,6 +1072,13 @@
             _PyUnicode_WSTR(unicode) = NULL;
         }
     }
+#ifdef Py_DEBUG
+    /* Fill the data with invalid characters to detect bugs earlier.
+       _PyUnicode_CheckConsistency(str, 1) detects invalid characters,
+       at least for ASCII and UCS-4 strings. U+00FF is invalid in ASCII
+       and U+FFFFFFFF is an invalid character in Unicode 6.0. */
+    memset(data, 0xff, size * kind);
+#endif
     assert(_PyUnicode_CheckConsistency((PyObject*)unicode, 0));
     return obj;
 }
diff --git a/Python/codecs.c b/Python/codecs.c
index 607feea..797a45f 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -534,6 +534,7 @@
         data = PyUnicode_DATA(res);
         for (i = 0; i < len; ++i)
             PyUnicode_WRITE(kind, data, i, '?');
+        assert(_PyUnicode_CheckConsistency(res, 1));
         return Py_BuildValue("(Nn)", res, end);
     }
     else if (PyObject_IsInstance(exc, PyExc_UnicodeDecodeError)) {
@@ -559,6 +560,7 @@
         data = PyUnicode_DATA(res);
         for (i=0; i < len; i++)
             PyUnicode_WRITE(kind, data, i, Py_UNICODE_REPLACEMENT_CHARACTER);
+        assert(_PyUnicode_CheckConsistency(res, 1));
         return Py_BuildValue("(Nn)", res, end);
     }
     else {
@@ -652,8 +654,8 @@
             }
             *outp++ = ';';
         }
-        restuple = Py_BuildValue("(On)", res, end);
-        Py_DECREF(res);
+        assert(_PyUnicode_CheckConsistency(res, 1));
+        restuple = Py_BuildValue("(Nn)", res, end);
         Py_DECREF(object);
         return restuple;
     }
@@ -720,8 +722,8 @@
             *outp++ = Py_hexdigits[c&0xf];
         }
 
-        restuple = Py_BuildValue("(On)", res, end);
-        Py_DECREF(res);
+        assert(_PyUnicode_CheckConsistency(res, 1));
+        restuple = Py_BuildValue("(Nn)", res, end);
         Py_DECREF(object);
         return restuple;
     }
diff --git a/Python/compile.c b/Python/compile.c
index 79d1d21..10e9ad2 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -263,6 +263,7 @@
         Py_DECREF(result);
         return NULL;
     }
+    assert(_PyUnicode_CheckConsistency(result, 1));
     return result;
 }
 
diff --git a/Python/import.c b/Python/import.c
index 8cf10e6..103e7de 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -992,6 +992,7 @@
                              (j = dot0-right));
     PyUnicode_WRITE(kind, data, i+j,   'p');
     PyUnicode_WRITE(kind, data, i+j+1, 'y');
+    assert(_PyUnicode_CheckConsistency(result, 1));
     return result;
 }