diff --git a/dlls/kernel32/tests/codepage.c b/dlls/kernel32/tests/codepage.c
index 71bedb7de70..54f62aecec5 100644
--- a/dlls/kernel32/tests/codepage.c
+++ b/dlls/kernel32/tests/codepage.c
@@ -276,12 +276,14 @@ static void test_overlapped_buffers(void)
 static void test_string_conversion(LPBOOL bUsedDefaultChar)
 {
     char mbc;
-    char mbs[5];
+    char mbs[15];
     int ret;
     WCHAR wc1 = 228;                           /* Western Windows-1252 character */
     WCHAR wc2 = 1088;                          /* Russian Windows-1251 character not displayable for Windows-1252 */
     static const WCHAR wcs[] = {'T', 'h', 1088, 'i', 0}; /* String with ASCII characters and a Russian character */
     static const WCHAR dbwcs[] = {28953, 25152, 0}; /* String with Chinese (codepage 950) characters */
+    static const WCHAR dbwcs2[] = {0x7bb8, 0x3d, 0xc813, 0xac00, 0xb77d, 0};
+    static const char default_char[] = {0xa3, 0xbf, 0};
 
     SetLastError(0xdeadbeef);
     ret = WideCharToMultiByte(1252, 0, &wc1, 1, &mbc, 1, NULL, bUsedDefaultChar);
@@ -358,6 +360,11 @@ static void test_string_conversion(LPBOOL bUsedDefaultChar)
     ok(!strcmp(mbs, "??"), "mbs is %s\n", mbs);
     if(bUsedDefaultChar) ok(*bUsedDefaultChar == TRUE, "bUsedDefaultChar is %d\n", *bUsedDefaultChar);
 
+    ret = WideCharToMultiByte(936, WC_COMPOSITECHECK, dbwcs2, -1, mbs, sizeof(mbs), (const char *)default_char, bUsedDefaultChar);
+    ok(ret == 10, "ret is %d\n", ret);
+    ok(!strcmp(mbs, "\xf3\xe7\x3d\xa3\xbf\xa3\xbf\xa3\xbf"), "mbs is %s\n", mbs);
+    if(bUsedDefaultChar) ok(*bUsedDefaultChar == TRUE, "bUsedDefaultChar is %d\n", *bUsedDefaultChar);
+
     /* Length-only tests */
     SetLastError(0xdeadbeef);
     ret = WideCharToMultiByte(1252, 0, &wc2, 1, NULL, 0, NULL, bUsedDefaultChar);
diff --git a/libs/wine/wctomb.c b/libs/wine/wctomb.c
index 3b081d45653..29a3d920142 100644
--- a/libs/wine/wctomb.c
+++ b/libs/wine/wctomb.c
@@ -275,6 +275,14 @@ static inline int is_valid_dbcs_mapping( const struct dbcs_table *table, int fla
     return 1;
 }
 
+/* compute the default char for the dbcs case */
+static inline WCHAR get_defchar_dbcs( const struct dbcs_table *table, const char *defchar )
+{
+    if (!defchar) return table->info.def_char;
+    if (!defchar[1]) return (unsigned char)defchar[0];
+    return ((unsigned char)defchar[0] << 8) | (unsigned char)defchar[1];
+}
+
 /* query necessary dst length for src string */
 static int get_length_dbcs( const struct dbcs_table *table, int flags,
                             const WCHAR *src, unsigned int srclen,
@@ -282,8 +290,7 @@ static int get_length_dbcs( const struct dbcs_table *table, int flags,
 {
     const unsigned short * const uni2cp_low = table->uni2cp_low;
     const unsigned short * const uni2cp_high = table->uni2cp_high;
-    WCHAR defchar_value = table->info.def_char;
-    WCHAR composed;
+    WCHAR defchar_value, composed;
     int len, tmp;
 
     if (!defchar && !used && !(flags & WC_COMPOSITECHECK))
@@ -295,7 +302,7 @@ static int get_length_dbcs( const struct dbcs_table *table, int flags,
         return len;
     }
 
-    if (defchar) defchar_value = defchar[1] ? ((defchar[0] << 8) | defchar[1]) : defchar[0];
+    defchar_value = get_defchar_dbcs( table, defchar );
     if (!used) used = &tmp;  /* avoid checking on every char */
     *used = 0;
     for (len = 0; srclen; len++, srclen--, src++)
@@ -376,11 +383,10 @@ static int wcstombs_dbcs_slow( const struct dbcs_table *table, int flags,
 {
     const unsigned short * const uni2cp_low = table->uni2cp_low;
     const unsigned short * const uni2cp_high = table->uni2cp_high;
-    WCHAR defchar_value = table->info.def_char;
+    WCHAR defchar_value = get_defchar_dbcs( table, defchar );
     WCHAR composed;
     int len, tmp;
 
-    if (defchar) defchar_value = defchar[1] ? ((defchar[0] << 8) | defchar[1]) : defchar[0];
     if (!used) used = &tmp;  /* avoid checking on every char */
     *used = 0;