diff --git a/rostests/apitests/crt/mbstowcs.c b/rostests/apitests/crt/mbstowcs.c
index 012fc7b748f..2c5f9fde952 100644
--- a/rostests/apitests/crt/mbstowcs.c
+++ b/rostests/apitests/crt/mbstowcs.c
@@ -12,14 +12,26 @@
 #include <specstrings.h>
 
 #define StrROS "ReactOS"
-
+#define LStrROS L"ReactOS"
 
 START_TEST(mbstowcs)
 {
     size_t len;
+    wchar_t out[ARRAYSIZE(LStrROS)];
 
-    len = mbstowcs(NULL, StrROS, sizeof(StrROS) / sizeof(StrROS[0]));
+    len = mbstowcs(NULL, StrROS, 0);
     ok(len == 7, "Got len = %u, excepting 7\n", len);
-    len = mbstowcs(NULL, StrROS, sizeof(StrROS) / sizeof(StrROS[0]) - 1);
+    len = mbstowcs(NULL, StrROS, 0);
     ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = mbstowcs(NULL, StrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = mbstowcs(NULL, StrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = mbstowcs(out, StrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    ok_wstr(out, LStrROS);
+    memset(out, 0, sizeof(out));
+    len = mbstowcs(out, StrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    ok_wstr(out, LStrROS);
 }
diff --git a/rostests/apitests/crt/wcstombs.c b/rostests/apitests/crt/wcstombs.c
index 1d21612e6eb..0297b2f96d6 100644
--- a/rostests/apitests/crt/wcstombs.c
+++ b/rostests/apitests/crt/wcstombs.c
@@ -11,15 +11,27 @@
 #include <stdlib.h>
 #include <specstrings.h>
 
-#define StrROS L"ReactOS"
-
+#define LStrROS L"ReactOS"
+#define StrROS "ReactOS"
 
 START_TEST(wcstombs)
 {
     size_t len;
+    char out[ARRAYSIZE(StrROS)];
 
-    len = wcstombs(NULL, StrROS, sizeof(StrROS) / sizeof(StrROS[0]));
+    len = wcstombs(NULL, LStrROS, 0);
     ok(len == 7, "Got len = %u, excepting 7\n", len);
-    len = wcstombs(NULL, StrROS, sizeof(StrROS) / sizeof(StrROS[0]) - 1);
+    len = wcstombs(NULL, LStrROS, 0);
     ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = wcstombs(NULL, LStrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = wcstombs(NULL, LStrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    len = wcstombs(out, LStrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    ok_str(out, StrROS);
+    memset(out, 0, sizeof(out));
+    len = wcstombs(out, LStrROS, ARRAYSIZE(out));
+    ok(len == 7, "Got len = %u, excepting 7\n", len);
+    ok_str(out, StrROS);
 }