diff --git a/src/calibre/devices/usbobserver/usbobserver.c b/src/calibre/devices/usbobserver/usbobserver.c
index 716ed61745..8165332cd3 100644
--- a/src/calibre/devices/usbobserver/usbobserver.c
+++ b/src/calibre/devices/usbobserver/usbobserver.c
@@ -372,7 +372,7 @@ usbobserver_has_mtp_interface(io_service_t usb_device) {
 
 static PyObject*
 usbobserver_is_mtp(PyObject *self, PyObject * args) {
-    int serial_sz = 0;
+    Py_ssize_t serial_sz = 0;
     long vendor_id = 0, product_id = 0, bcd = 0;
     char *serial = NULL, buf[500] = {0};
     CFNumberRef num = NULL;
diff --git a/src/calibre/utils/lzx/lzxmodule.c b/src/calibre/utils/lzx/lzxmodule.c
index bb18277ccf..150c55bd46 100644
--- a/src/calibre/utils/lzx/lzxmodule.c
+++ b/src/calibre/utils/lzx/lzxmodule.c
@@ -152,7 +152,7 @@ decompress(PyObject *self, PyObject *args)
 {
     unsigned char *inbuf;
     unsigned char *outbuf;
-    unsigned int inlen;
+    Py_ssize_t inlen;
     unsigned int outlen;
     int err;
     memory_file source;
diff --git a/src/calibre/utils/msdes/msdesmodule.c b/src/calibre/utils/msdes/msdesmodule.c
index a69ff19ea2..285f359d3a 100644
--- a/src/calibre/utils/msdes/msdesmodule.c
+++ b/src/calibre/utils/msdes/msdesmodule.c
@@ -17,7 +17,7 @@ static PyObject *
 msdes_deskey(PyObject *self, PyObject *args)
 {
     unsigned char *key = NULL;
-    unsigned int len = 0;
+    Py_ssize_t len = 0;
     short int edf = 0;
 
     if (!PyArg_ParseTuple(args, "y#h", &key, &len, &edf)) {
@@ -44,8 +44,8 @@ msdes_des(PyObject *self, PyObject *args)
 {
     unsigned char *inbuf = NULL;
     unsigned char *outbuf = NULL;
-    unsigned int len = 0;
-    unsigned int off = 0;
+    Py_ssize_t len = 0;
+    Py_ssize_t off = 0;
     PyObject *retval = NULL;
 
     if (!PyArg_ParseTuple(args, "y#", &inbuf, &len)) {
diff --git a/src/calibre/utils/podofo/doc.cpp b/src/calibre/utils/podofo/doc.cpp
index fea2a21c68..8af45e6214 100644
--- a/src/calibre/utils/podofo/doc.cpp
+++ b/src/calibre/utils/podofo/doc.cpp
@@ -376,7 +376,7 @@ error:
 static PyObject *
 PDFDoc_set_xmp_metadata(PDFDoc *self, PyObject *args) {
     const char *raw = NULL;
-    long len = 0;
+    Py_ssize_t len = 0;
     PoDoFo::PdfObject *metadata = NULL, *catalog = NULL;
     PoDoFo::PdfStream *str = NULL;
     TVecFilters compressed(1);
diff --git a/src/calibre/utils/windows/winutil.cpp b/src/calibre/utils/windows/winutil.cpp
index a32d610f9a..c9696011e7 100644
--- a/src/calibre/utils/windows/winutil.cpp
+++ b/src/calibre/utils/windows/winutil.cpp
@@ -959,7 +959,8 @@ is_wow64_process(PyObject *self, PyObject *args) {
 
 static PyObject*
 write_file(PyObject *self, PyObject *args) {
-    int size, offset = 0;
+    int offset = 0;
+    Py_ssize_t size;
     const char *data;
     HANDLE handle;
     if (!PyArg_ParseTuple(args, "O&y#|i", convert_handle, &handle, &data, &size, &offset)) return NULL;