diff --git a/src/calibre/utils/windows/winutil.cpp b/src/calibre/utils/windows/winutil.cpp
index 612f7d085e..df15066004 100644
--- a/src/calibre/utils/windows/winutil.cpp
+++ b/src/calibre/utils/windows/winutil.cpp
@@ -773,13 +773,18 @@ get_long_path_name(PyObject *self, PyObject *args) {
     DWORD current_size = 4096;
     wchar_raii buf((wchar_t*)PyMem_Malloc(current_size * sizeof(wchar_t)));
     if (!buf) return PyErr_NoMemory();
-    DWORD needed_size = GetLongPathNameW(path.ptr(), buf.ptr(), current_size);
+    DWORD needed_size;
+    Py_BEGIN_ALLOW_THREADS
+    needed_size = GetLongPathNameW(path.ptr(), buf.ptr(), current_size);
+    Py_END_ALLOW_THREADS
     if (needed_size >= current_size - 32) {
         current_size = needed_size + 32;
         PyMem_Free(buf.ptr());
         buf.set_ptr((wchar_t*)PyMem_Malloc(current_size * sizeof(wchar_t)));
         if (!buf) return PyErr_NoMemory();
+        Py_BEGIN_ALLOW_THREADS
         needed_size = GetLongPathNameW(path.ptr(), buf.ptr(), current_size);
+        Py_END_ALLOW_THREADS
     }
     if (!needed_size) return PyErr_SetExcFromWindowsErrWithFilenameObject(PyExc_OSError, 0, PyTuple_GET_ITEM(args, 0));
     if (needed_size >= current_size - 2) {