diff --git a/lib/curlx/fopen.c b/lib/curlx/fopen.c
index 37ca02671a..976eff9e2b 100644
--- a/lib/curlx/fopen.c
+++ b/lib/curlx/fopen.c
@@ -68,7 +68,7 @@ static wchar_t *fn_convert_UTF8_to_wchar(const char *str_utf8)
     if(str_w_len > 0) {
       str_w = CURLX_MALLOC(str_w_len * sizeof(wchar_t));
       if(str_w) {
-        if(MultiByteToWideChar(CP_UTF8, 0,
+        if(MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS,
                                str_utf8, -1, str_w, str_w_len) == 0) {
           CURLX_FREE(str_w);
           return NULL;
diff --git a/lib/curlx/multibyte.c b/lib/curlx/multibyte.c
index 715d2b8dc2..4ee39d962a 100644
--- a/lib/curlx/multibyte.c
+++ b/lib/curlx/multibyte.c
@@ -41,8 +41,8 @@ wchar_t *curlx_convert_UTF8_to_wchar(const char *str_utf8)
     if(str_w_len > 0) {
       str_w = curlx_malloc(str_w_len * sizeof(wchar_t));
       if(str_w) {
-        if(MultiByteToWideChar(CP_UTF8, 0, str_utf8, -1, str_w,
-                               str_w_len) == 0) {
+        if(MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS,
+                               str_utf8, -1, str_w, str_w_len) == 0) {
           curlx_free(str_w);
           return NULL;
         }
diff --git a/lib/idn.c b/lib/idn.c
index b26f251d97..943a520eff 100644
--- a/lib/idn.c
+++ b/lib/idn.c
@@ -172,7 +172,8 @@ static CURLcode win32_idn_to_ascii(const char *in, char **out)
   /* Returned in_w_len includes the null-terminator, which then gets
      preserved across the calls that follow, ending up terminating
      the buffer returned to the caller. */
-  in_w_len = MultiByteToWideChar(CP_UTF8, 0, in, -1, in_w, IDN_MAX_LENGTH);
+  in_w_len = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS,
+                                 in, -1, in_w, IDN_MAX_LENGTH);
   if(in_w_len) {
     wchar_t punycode[IDN_MAX_LENGTH];
     int chars = IdnToAscii(0, in_w, in_w_len, punycode, IDN_MAX_LENGTH);
@@ -198,7 +199,8 @@ static CURLcode win32_ascii_to_idn(const char *in, char **out)
   /* Returned in_w_len includes the null-terminator, which then gets
      preserved across the calls that follow, ending up terminating
      the buffer returned to the caller. */
-  in_w_len = MultiByteToWideChar(CP_UTF8, 0, in, -1, in_w, IDN_MAX_LENGTH);
+  in_w_len = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS,
+                                 in, -1, in_w, IDN_MAX_LENGTH);
   if(in_w_len) {
     WCHAR idn[IDN_MAX_LENGTH]; /* stores a UTF-16 string */
     int chars = IdnToUnicode(0, in_w, in_w_len, idn, IDN_MAX_LENGTH);
diff --git a/lib/vtls/schannel.c b/lib/vtls/schannel.c
index d322c755e8..2b8166de12 100644
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -473,10 +473,8 @@ static CURLcode get_client_cert(struct Curl_cfilter *cf,
       if(pszPassword) {
         int str_w_len = 0;
         if(pwd_len > 0)
-          str_w_len = MultiByteToWideChar(CP_UTF8,
-                                          MB_ERR_INVALID_CHARS,
-                                          sslc->key_passwd,
-                                          (int)pwd_len,
+          str_w_len = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS,
+                                          sslc->key_passwd, (int)pwd_len,
                                           pszPassword, (int)(pwd_len + 1));
 
         if((str_w_len >= 0) && (str_w_len <= (int)pwd_len))