diff --git a/dlls/msvcrt/mbcs.c b/dlls/msvcrt/mbcs.c
index d30739900c1..5d389bf3686 100644
--- a/dlls/msvcrt/mbcs.c
+++ b/dlls/msvcrt/mbcs.c
@@ -2266,6 +2266,11 @@ MSVCRT_size_t CDECL MSVCRT__mbstowcs_l(MSVCRT_wchar_t *wcstr, const char *mbstr,
 
     size = MultiByteToWideChar(locinfo->lc_codepage, 0,
             mbstr, size, wcstr, count);
+    if(!size) {
+        if(count) wcstr[0] = '\0';
+        *MSVCRT__errno() = MSVCRT_EILSEQ;
+        return -1;
+    }
 
     if(size<count && wcstr)
         wcstr[size] = '\0';