diff --git a/src/os/windows/win32.cpp b/src/os/windows/win32.cpp
index 073f70b98c..ddcfd4866c 100644
--- a/src/os/windows/win32.cpp
+++ b/src/os/windows/win32.cpp
@@ -247,7 +247,7 @@ bool FiosGetDiskFreeSpace(const char *path, uint64 *tot)
 
 	ULARGE_INTEGER bytes_free;
 	bool retval = GetDiskFreeSpaceEx(OTTD2FS(path).c_str(), &bytes_free, nullptr, nullptr);
-	if (retval) *tot = bytes_free.QuadPart;
+	if (retval && tot != nullptr) *tot = bytes_free.QuadPart;
 
 	SetErrorMode(sem); // reset previous setting
 	return retval;