diff --git a/src/calibre/devices/mtp/unix/libmtp.c b/src/calibre/devices/mtp/unix/libmtp.c
index 21db96f115..58e8df6686 100644
--- a/src/calibre/devices/mtp/unix/libmtp.c
+++ b/src/calibre/devices/mtp/unix/libmtp.c
@@ -7,15 +7,17 @@
 #include "devices.h"
 
 // Macros and utilities
+static PyObject *MTPError = NULL;
+
 #define ENSURE_DEV(rval) \
     if (self->device == NULL) { \
-        PyErr_SetString(PyExc_ValueError, "This device has not been initialized."); \
+        PyErr_SetString(MTPError, "This device has not been initialized."); \
         return rval; \
     }
 
 #define ENSURE_STORAGE(rval) \
     if (self->device->storage == NULL) { \
-        PyErr_SetString(PyExc_RuntimeError, "The device has no storage information."); \
+        PyErr_SetString(MTPError, "The device has no storage information."); \
         return rval; \
     }
 
@@ -31,6 +33,7 @@
 #define AC_ReadOnly             0x0001
 #define AC_ReadOnly_with_Object_Deletion    0x0002
 
+
 typedef struct {
     PyObject *obj;
     PyObject *extra;
@@ -183,7 +186,7 @@ libmtp_Device_init(libmtp_Device *self, PyObject *args, PyObject *kwds)
     Py_END_ALLOW_THREADS;
 
     if (dev == NULL) { 
-        PyErr_SetString(PyExc_ValueError, "Unable to open raw device."); 
+        PyErr_SetString(MTPError, "Unable to open raw device."); 
         return -1;
     }
 
@@ -272,7 +275,7 @@ static PyObject*
 libmtp_Device_update_storage_info(libmtp_Device *self, PyObject *args, PyObject *kwargs) {
     ENSURE_DEV(NULL);
     if (LIBMTP_Get_Storage(self->device, LIBMTP_STORAGE_SORTBY_NOTSORTED) < 0) {
-        PyErr_SetString(PyExc_RuntimeError, "Failed to get storage infor for device.");
+        PyErr_SetString(MTPError, "Failed to get storage infor for device.");
         return NULL;
     }
     Py_RETURN_NONE;
@@ -761,6 +764,8 @@ initlibmtp(void) {
     
     m = Py_InitModule3("libmtp", libmtp_methods, "Interface to libmtp.");
     if (m == NULL) return;
+    MTPError = PyErr_NewException("libmtp.MTPError", NULL, NULL);
+    if (MTPError == NULL) return;
 
     LIBMTP_Init();
     LIBMTP_Set_Debug(LIBMTP_DEBUG_NONE);
diff --git a/src/calibre/devices/mtp/windows/wpd.cpp b/src/calibre/devices/mtp/windows/wpd.cpp
index f4029e7061..889a3e02c5 100644
--- a/src/calibre/devices/mtp/windows/wpd.cpp
+++ b/src/calibre/devices/mtp/windows/wpd.cpp
@@ -14,7 +14,7 @@
 #include <PortableDeviceApi.h>
 
 static int _com_initialized = 0;
-static PyObject *WPDError;
+static PyObject *WPDError = NULL;
 static IPortableDeviceManager *portable_device_manager = NULL;
 
 static PyObject *
@@ -32,6 +32,7 @@ wpd_init(PyObject *self, PyObject *args) {
                 CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&portable_device_manager));
 
         if (FAILED(hr)) {
+            portable_device_manager = NULL;
             PyErr_SetString(WPDError, (hr == REGDB_E_CLASSNOTREG) ? 
                 "This computer is not running the Windows Portable Device framework. You may need to install Windows Media Player 11 or newer." : 
                 "Failed to create the WPD device manager interface");