diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index 7c883ba..5b860dd 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -700,6 +700,9 @@
         self.assertRaisesRegex(ValueError, 'Cannot specify both', format, 3, '_,d')
         self.assertRaisesRegex(ValueError, 'Cannot specify both', format, 3, ',_d')
 
+        self.assertRaisesRegex(ValueError, "Cannot specify ',' with 's'", format, 3, ',s')
+        self.assertRaisesRegex(ValueError, "Cannot specify '_' with 's'", format, 3, '_s')
+
         # ensure that only int and float type specifiers work
         for format_spec in ([chr(x) for x in range(ord('a'), ord('z')+1)] +
                             [chr(x) for x in range(ord('A'), ord('Z')+1)]):
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 71e673d..ba09cc6 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -28,16 +28,17 @@
 }
 
 static void
-invalid_comma_type(Py_UCS4 presentation_type)
+invalid_thousands_separator_type(char specifier, Py_UCS4 presentation_type)
 {
+    assert(specifier == ',' || specifier == '_');
     if (presentation_type > 32 && presentation_type < 128)
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '%c'.",
-                     (char)presentation_type);
+                     "Cannot specify '%c' with '%c'.",
+                     specifier, (char)presentation_type);
     else
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '\\x%x'.",
-                     (unsigned int)presentation_type);
+                     "Cannot specify '%c' with '\\x%x'.",
+                     specifier, (unsigned int)presentation_type);
 }
 
 static void
@@ -117,8 +118,8 @@
 /* Locale type codes. LT_NO_LOCALE must be zero. */
 enum LocaleType {
     LT_NO_LOCALE = 0,
-    LT_DEFAULT_LOCALE,
-    LT_UNDERSCORE_LOCALE,
+    LT_DEFAULT_LOCALE = ',',
+    LT_UNDERSCORE_LOCALE = '_',
     LT_UNDER_FOUR_LOCALE,
     LT_CURRENT_LOCALE
 };
@@ -314,7 +315,7 @@
             }
             /* fall through */
         default:
-            invalid_comma_type(format->type);
+            invalid_thousands_separator_type(format->thousands_separators, format->type);
             return 0;
         }
     }