diff --git a/dlls/ntdll/unix/env.c b/dlls/ntdll/unix/env.c
index 52736344078..13bf85d2eb6 100644
--- a/dlls/ntdll/unix/env.c
+++ b/dlls/ntdll/unix/env.c
@@ -644,10 +644,11 @@ DWORD ntdll_umbstowcs( const char *src, DWORD srclen, WCHAR *dst, DWORD dstlen )
     }
     else  /* utf-8 */
     {
+        reslen = 0;
         RtlUTF8ToUnicodeN( dst, dstlen * sizeof(WCHAR), &reslen, src, srclen );
         reslen /= sizeof(WCHAR);
 #ifdef __APPLE__  /* work around broken Mac OS X filesystem that enforces NFD */
-        if (reslen && nfc_table) reslen = compose_string( nfc_table, dst - reslen, reslen );
+        if (reslen && nfc_table) reslen = compose_string( nfc_table, dst, reslen );
 #endif
     }
     return reslen;