diff --git a/Modules/fcntlmodule.c b/Modules/fcntlmodule.c
index e6a40ffc5a26144..5dd3df9bb408f0c 100644
--- a/Modules/fcntlmodule.c
+++ b/Modules/fcntlmodule.c
@@ -121,13 +121,14 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
             return PyBytes_FromStringAndSize(buf, len);
         }
         else {
-            PyBytesWriter *writer = PyBytesWriter_Create(len);
+            PyBytesWriter *writer = PyBytesWriter_Create(len + GUARDSZ);
             if (writer == NULL) {
                 PyBuffer_Release(&view);
                 return NULL;
             }
             char *ptr = PyBytesWriter_GetData(writer);
             memcpy(ptr, view.buf, len);
+            memcpy(ptr + len, guard, GUARDSZ);
             PyBuffer_Release(&view);
 
             do {
@@ -142,7 +143,7 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }
-            if (ptr[len] != '\0') {
+            if (memcmp(ptr + len, guard, GUARDSZ) != 0) {
                 PyErr_SetString(PyExc_SystemError,
                         "Memory corruption in fcntl() due to "
                         "buffer overflow. "
@@ -151,7 +152,8 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }
-            return PyBytesWriter_Finish(writer);
+            // Truncate the trailing guard bytes
+            return PyBytesWriter_FinishWithSize(writer, len);
         }
 #undef FCNTL_BUFSZ
     }
@@ -316,13 +318,14 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
             return PyBytes_FromStringAndSize(buf, len);
         }
         else {
-            PyBytesWriter *writer = PyBytesWriter_Create(len);
+            PyBytesWriter *writer = PyBytesWriter_Create(len + GUARDSZ);
             if (writer == NULL) {
                 PyBuffer_Release(&view);
                 return NULL;
             }
             char *ptr = PyBytesWriter_GetData(writer);
             memcpy(ptr, view.buf, len);
+            memcpy(ptr + len, guard, GUARDSZ);
             PyBuffer_Release(&view);
 
             do {
@@ -337,7 +340,7 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }
-            if (ptr[len] != '\0') {
+            if (memcmp(ptr + len, guard, GUARDSZ) != 0) {
                 PyErr_SetString(PyExc_SystemError,
                         "Memory corruption in ioctl() due to "
                         "buffer overflow. "
@@ -346,7 +349,8 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }
-            return PyBytesWriter_Finish(writer);
+            // Truncate the trailing guard bytes
+            return PyBytesWriter_FinishWithSize(writer, len);
         }
 #undef IOCTL_BUFSZ
     }