diff --git a/setup/installer/windows/file_dialogs.cpp b/setup/installer/windows/file_dialogs.cpp
index 43e3d488fc..5ea61a01a5 100644
--- a/setup/installer/windows/file_dialogs.cpp
+++ b/setup/installer/windows/file_dialogs.cpp
@@ -94,7 +94,7 @@ static void print_com_error(HRESULT hr, const char *msg) {
     int sz = 0;
     const char *buf = to_utf8(emsg, &sz);
     if (buf == NULL) { fprintf(stderr, "%s", msg); }
-    else { fprintf(stderr, "%s: (HRESULT=0x%x) %s\n", msg, hr, emsg); }
+    else { fprintf(stderr, "%s: (HRESULT=0x%x) %s\n", msg, hr, buf); }
     fflush(stderr);
 }
 
@@ -188,7 +188,7 @@ int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine
 	HWND parent = NULL;
 	bool save_dialog = false, multiselect = false, confirm_overwrite = false, only_dirs = false, no_symlinks = false;
 	unsigned short len = 0;
-	LPWSTR title = NULL, folder = NULL, filename = NULL, save_path = NULL;
+	LPWSTR title = NULL, folder = NULL, filename = NULL, save_path = NULL, echo = NULL;
     COMDLG_FILTERSPEC *file_types = NULL;
     UINT num_file_types = 0;
 
@@ -208,7 +208,7 @@ int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine
 			READ(sizeof(HWND), buf);
 			if (sizeof(HWND) == 8) parent = (HWND)*((__int64*)buf);
 			else if (sizeof(HWND) == 4) parent = (HWND)*((__int32*)buf);
-			else { fprintf(stderr, "Unknown pointer size: %d", sizeof(HWND)); fflush(stderr); return 1;}
+			else { fprintf(stderr, "Unknown pointer size: %zd", sizeof(HWND)); fflush(stderr); return 1;}
 		}
 
 		else if CHECK_KEY("TITLE") { READSTR(title) }
@@ -231,11 +231,20 @@ int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine
 
         else if CHECK_KEY("FILE_TYPES") { file_types = read_file_types(&num_file_types); if (file_types == NULL) return 1; }
 
+        else if CHECK_KEY("ECHO") { READSTR(echo) }
+
 		else {
 			PRINTERR("Unknown key");
 			return 1;
 		}
 	}
 
+    if (echo != NULL) { 
+        int echo_sz = 0;
+        char *echo_buf = to_utf8(echo, &echo_sz);
+        fprintf(stdout, "%s", echo_buf); 
+        return 0; 
+    }
+
 	return show_dialog(parent, save_dialog, title, folder, filename, save_path, multiselect, confirm_overwrite, only_dirs, no_symlinks, file_types, num_file_types);
 }
diff --git a/src/calibre/gui2/win_file_dialogs.py b/src/calibre/gui2/win_file_dialogs.py
index bf9938b286..8e91343cd6 100644
--- a/src/calibre/gui2/win_file_dialogs.py
+++ b/src/calibre/gui2/win_file_dialogs.py
@@ -151,8 +151,18 @@ def run_file_dialog(
     ans = tuple((os.path.abspath(x.decode('utf-8')) for x in h.stdoutdata.split(b'\0') if x))
     return ans
 
+def test():
+    p = subprocess.Popen([HELPER], stdout=subprocess.PIPE, stdin=subprocess.PIPE, stderr=subprocess.PIPE)
+    echo = '\U0001f431 Hello world!'
+    stdout, stderr = p.communicate(serialize_string('ECHO', echo))
+    if p.wait() != 0:
+        raise Exception('File dialog failed: ' + stderr.decode('utf-8'))
+    if stdout.decode('utf-8') != echo:
+        raise RuntimeError('Unexpected response: %s' % stdout.decode('utf-8'))
+
 if __name__ == '__main__':
     HELPER = sys.argv[-1]
+    test()
     app = QApplication([])
     q = QMainWindow()
     _ = lambda x: x
diff --git a/src/calibre/test_build.py b/src/calibre/test_build.py
index 51bdf93cad..ed99d09bf6 100644
--- a/src/calibre/test_build.py
+++ b/src/calibre/test_build.py
@@ -177,6 +177,11 @@ def test_imaging():
         raise RuntimeError('PIL choked!')
     fprint('PIL OK!')
 
+def test_file_dialog_helper():
+    from calibre.gui2.win_file_dialogs import test
+    test()
+    print('File dialog helper OK!')
+
 def test_unrar():
     from calibre.utils.unrar import test_basic
     test_basic()
@@ -304,6 +309,7 @@ def test():
     if iswindows:
         test_wpd()
         test_winutil()
+        test_file_dialog_helper()
     else:
         test_terminal()
     if isosx: