diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1777,20 +1777,9 @@ class StringModuleTest(unittest.TestCase
                          b' 3.14 ')
         self.assertRaises(UnicodeEncodeError,
                           unicode_encodedecimal, "123\u20ac", "strict")
-        self.assertEqual(unicode_encodedecimal("123\u20ac", "replace"),
-                         b'123?')
-        self.assertEqual(unicode_encodedecimal("123\u20ac", "ignore"),
-                         b'123')
-        self.assertEqual(unicode_encodedecimal("123\u20ac", "xmlcharrefreplace"),
-                         b'123€')
-        self.assertEqual(unicode_encodedecimal("123\u20ac", "backslashreplace"),
-                         b'123\\u20ac')
-        self.assertEqual(unicode_encodedecimal("123\u20ac\N{EM SPACE}", "replace"),
-                         b'123? ')
-        self.assertEqual(unicode_encodedecimal("123\u20ac\u20ac", "replace"),
-                         b'123??')
-        self.assertEqual(unicode_encodedecimal("123\u20ac\u0660", "replace"),
-                         b'123?0')
+        self.assertRaisesRegex(
+            ValueError, "decimal encoder does not support replace error handler",
+            unicode_encodedecimal, "123\u20ac", "replace")
 
     def test_transform_decimal(self):
         from _testcapi import unicode_transformdecimaltoascii as transform_decimal
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@ What's New in Python 3.2.3?
 Core and Builtins
 -----------------
 
+- Issue #13093: PyUnicode_EncodeDecimal() doesn't support error handlers
+  different than "strict" anymore. The caller was unable to compute the
+  size of the output buffer: it depends on the error handler.
+
 - Issue #13338: Handle all enumerations in _Py_ANNOTATE_MEMORY_ORDER
   to allow compiling extension modules with -Wswitch-enum on gcc.
   Initial patch by Floris Bruynooghe.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -6280,30 +6280,26 @@ int PyUnicode_EncodeDecimal(Py_UNICODE *
                             const char *errors)
 {
     Py_UNICODE *p, *end;
-    PyObject *errorHandler = NULL;
-    PyObject *exc = NULL;
-    const char *encoding = "decimal";
-    const char *reason = "invalid decimal Unicode string";
-    /* the following variable is used for caching string comparisons
-     * -1=not initialized, 0=unknown, 1=strict, 2=replace, 3=ignore, 4=xmlcharrefreplace */
-    int known_errorHandler = -1;
 
     if (output == NULL) {
         PyErr_BadArgument();
         return -1;
     }
 
+    if (errors != NULL && strcmp(errors, "strict") != 0) {
+         PyErr_Format(PyExc_ValueError,
+                      "decimal encoder does not support %s error handler",
+                      errors);
+         return -1;
+    }
+
     p = s;
     end = s + length;
     while (p < end) {
         register Py_UNICODE ch = *p;
         int decimal;
-        PyObject *repunicode;
-        Py_ssize_t repsize;
-        Py_ssize_t newpos;
-        Py_UNICODE *uni2;
-        Py_UNICODE *collstart;
-        Py_UNICODE *collend;
+        Py_ssize_t startpos;
+        PyObject *errorHandler, *exc;
 
         if (Py_UNICODE_ISSPACE(ch)) {
             *output++ = ' ';
@@ -6322,90 +6318,19 @@ int PyUnicode_EncodeDecimal(Py_UNICODE *
             continue;
         }
         /* All other characters are considered unencodable */
-        collstart = p;
-        for (collend = p+1; collend < end; collend++) {
-            if ((0 < *collend && *collend < 256) ||
-                Py_UNICODE_ISSPACE(*collend) ||
-                0 <= Py_UNICODE_TODECIMAL(*collend))
-                break;
-        }
-        /* cache callback name lookup
-         * (if not done yet, i.e. it's the first error) */
-        if (known_errorHandler==-1) {
-            if ((errors==NULL) || (!strcmp(errors, "strict")))
-                known_errorHandler = 1;
-            else if (!strcmp(errors, "replace"))
-                known_errorHandler = 2;
-            else if (!strcmp(errors, "ignore"))
-                known_errorHandler = 3;
-            else if (!strcmp(errors, "xmlcharrefreplace"))
-                known_errorHandler = 4;
-            else
-                known_errorHandler = 0;
-        }
-        switch (known_errorHandler) {
-        case 1: /* strict */
-            raise_encode_exception(&exc, encoding, s, length, collstart-s, collend-s, reason);
-            goto onError;
-        case 2: /* replace */
-            for (p = collstart; p < collend; ++p)
-                *output++ = '?';
-            /* fall through */
-        case 3: /* ignore */
-            p = collend;
-            break;
-        case 4: /* xmlcharrefreplace */
-            /* generate replacement (temporarily (mis)uses p) */
-            for (p = collstart; p < collend; ++p)
-                output += sprintf(output, "&#%d;", (int)*p);
-            p = collend;
-            break;
-        default:
-            repunicode = unicode_encode_call_errorhandler(errors, &errorHandler,
-                                                          encoding, reason, s, length, &exc,
-                                                          collstart-s, collend-s, &newpos);
-            if (repunicode == NULL)
-                goto onError;
-            if (!PyUnicode_Check(repunicode)) {
-                /* Byte results not supported, since they have no decimal property. */
-                PyErr_SetString(PyExc_TypeError, "error handler should return unicode");
-                Py_DECREF(repunicode);
-                goto onError;
-            }
-            /* generate replacement  */
-            repsize = PyUnicode_GET_SIZE(repunicode);
-            for (uni2 = PyUnicode_AS_UNICODE(repunicode); repsize-->0; ++uni2) {
-                Py_UNICODE ch = *uni2;
-                if (Py_UNICODE_ISSPACE(ch))
-                    *output++ = ' ';
-                else {
-                    decimal = Py_UNICODE_TODECIMAL(ch);
-                    if (decimal >= 0)
-                        *output++ = '0' + decimal;
-                    else if (0 < ch && ch < 256)
-                        *output++ = (char)ch;
-                    else {
-                        Py_DECREF(repunicode);
-                        raise_encode_exception(&exc, encoding,
-                                               s, length, collstart-s, collend-s, reason);
-                        goto onError;
-                    }
-                }
-            }
-            p = s + newpos;
-            Py_DECREF(repunicode);
-        }
+        startpos = p-s;
+        errorHandler = NULL;
+        exc = NULL;
+        raise_encode_exception(&exc, "decimal", s, length,
+                               startpos, startpos+1,
+                               "invalid decimal Unicode string");
+        Py_XDECREF(exc);
+        Py_XDECREF(errorHandler);
+        return -1;
     }
     /* 0-terminate the output string */
     *output++ = '\0';
-    Py_XDECREF(exc);
-    Py_XDECREF(errorHandler);
     return 0;
-
-  onError:
-    Py_XDECREF(exc);
-    Py_XDECREF(errorHandler);
-    return -1;
 }
 
 /* --- Helpers ------------------------------------------------------------ */