NtUserDefSetText() in Windows 10 will panic if you set the ansi flag incorrectly.

DefText.c

#include <windows.h>
#include <winternl.h>
#include <stdio.h>

#pragma comment(lib, "user32")
#pragma comment(lib, "gdi32")

typedef struct _LARGE_STRING {
    ULONG Length;
    ULONG MaximumLength:31;
    ULONG bAnsi:1;
    PVOID Buffer;
} LARGE_STRING, *PLARGE_STRING;

static CHAR kWindowText[32] = "Hello World";

int main(int argc, char **argv) {
    FARPROC NtUserDefSetText = GetProcAddress(LoadLibrary("WIN32U"), "NtUserDefSetText");
    WNDCLASSEX WindowClass = {0};
    HWND Window;
    LARGE_STRING DefText = {
        .Length         = sizeof kWindowText,
        .MaximumLength  = sizeof kWindowText,
        .bAnsi          = FALSE,
        .Buffer         = kWindowText,
    };

    // This string has bAnsi set to FALSE, so an odd Length is impossible (must be a count of WCHARs)
    // Unless you set the flag incorrectly..
    DefText.MaximumLength   |= 1;
    DefText.Length          |= 1;

    WindowClass.cbSize         = sizeof(WNDCLASSEX);
    WindowClass.lpfnWndProc    = DefWindowProc;
    WindowClass.hInstance      = GetModuleHandle(NULL);
    WindowClass.lpszClassName  = "Class";

    RegisterClassEx(&WindowClass);

    Window = CreateWindowEx(0, "Class", "Window", 0, CW_USEDEFAULT, 0, 128, 128, NULL, NULL, GetModuleHandle(NULL), NULL);

    NtUserDefSetText(Window, &DefText);
    return 1;
}