diff --git a/Include/fileobject.h b/Include/fileobject.h
index ebbb521d17b..9fd0646be47 100644
--- a/Include/fileobject.h
+++ b/Include/fileobject.h
@@ -57,6 +57,11 @@ PyAPI_DATA(const char *) Py_FileSystemDefaultEncoding;
 char *Py_UniversalNewlineFgets(char *, int, FILE*, PyObject *);
 size_t Py_UniversalNewlineFread(char *, size_t, FILE *, PyObject *);
 
+/* A routine to do sanity checking on the file mode string.  returns
+   non-zero on if an exception occurred
+*/
+int _PyFile_SanitizeMode(char *mode);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 158b12e2eeb..b984c3004a1 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6170,16 +6170,23 @@ static PyObject *
 posix_fdopen(PyObject *self, PyObject *args)
 {
 	int fd;
-	char *mode = "r";
+	char *orgmode = "r";
 	int bufsize = -1;
 	FILE *fp;
 	PyObject *f;
-	if (!PyArg_ParseTuple(args, "i|si", &fd, &mode, &bufsize))
+	char *mode;
+	if (!PyArg_ParseTuple(args, "i|si", &fd, &orgmode, &bufsize))
 		return NULL;
 
-	if (mode[0] != 'r' && mode[0] != 'w' && mode[0] != 'a') {
-		PyErr_Format(PyExc_ValueError,
-			     "invalid file mode '%s'", mode);
+	/* Sanitize mode.  See fileobject.c */
+	mode = PyMem_MALLOC(strlen(orgmode)+3);
+	if (!mode) {
+		PyErr_NoMemory();
+		return NULL;
+	}
+	strcpy(mode, orgmode);
+	if (_PyFile_SanitizeMode(mode)) {
+		PyMem_FREE(mode);
 		return NULL;
 	}
 	Py_BEGIN_ALLOW_THREADS
@@ -6200,10 +6207,11 @@ posix_fdopen(PyObject *self, PyObject *args)
 #else
 	fp = fdopen(fd, mode);
 #endif
+	PyMem_FREE(mode);
 	Py_END_ALLOW_THREADS
 	if (fp == NULL)
 		return posix_error();
-	f = PyFile_FromFile(fp, "<fdopen>", mode, fclose);
+	f = PyFile_FromFile(fp, "<fdopen>", orgmode, fclose);
 	if (f != NULL)
 		PyFile_SetBufSize(f, bufsize);
 	return f;
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 869ded916cb..f954e36fa59 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -139,17 +139,16 @@ fill_file_fields(PyFileObject *f, FILE *fp, PyObject *name, char *mode,
    ignore stuff they don't understand... write or append mode with
    universal newline support is expressly forbidden by PEP 278.
    Additionally, remove the 'U' from the mode string as platforms
-   won't know what it is. */
-/* zero return is kewl - one is un-kewl */
-static int
-sanitize_the_mode(char *mode)
+   won't know what it is. Non-zero return signals an exception */
+int
+_PyFile_SanitizeMode(char *mode)
 {
 	char *upos;
 	size_t len = strlen(mode);
 
 	if (!len) {
 		PyErr_SetString(PyExc_ValueError, "empty mode string");
-		return 1;
+		return -1;
 	}
 
 	upos = strchr(mode, 'U');
@@ -160,7 +159,7 @@ sanitize_the_mode(char *mode)
 			PyErr_Format(PyExc_ValueError, "universal newline "
 			             "mode can only be used with modes "
 				     "starting with 'r'");
-			return 1;
+			return -1;
 		}
 
 		if (mode[0] != 'r') {
@@ -175,7 +174,7 @@ sanitize_the_mode(char *mode)
 	} else if (mode[0] != 'r' && mode[0] != 'w' && mode[0] != 'a') {
 		PyErr_Format(PyExc_ValueError, "mode string must begin with "
 	        	    "one of 'r', 'w', 'a' or 'U', not '%.200s'", mode);
-		return 1;
+		return -1;
 	}
 
 	return 0;
@@ -204,7 +203,7 @@ open_the_file(PyFileObject *f, char *name, char *mode)
 	}
 	strcpy(newmode, mode);
 
-	if (sanitize_the_mode(newmode)) {
+	if (_PyFile_SanitizeMode(newmode)) {
 		f = NULL;
 		goto cleanup;
 	}