diff --git a/Include/fileobject.h b/Include/fileobject.h
index 94d3591..632c436 100644
--- a/Include/fileobject.h
+++ b/Include/fileobject.h
@@ -21,7 +21,7 @@
 extern DL_IMPORT(PyObject *) PyFile_GetLine(PyObject *, int);
 extern DL_IMPORT(int) PyFile_WriteObject(PyObject *, PyObject *, int);
 extern DL_IMPORT(int) PyFile_SoftSpace(PyObject *, int);
-extern DL_IMPORT(int) PyFile_WriteString(char *, PyObject *);
+extern DL_IMPORT(int) PyFile_WriteString(const char *, PyObject *);
 extern DL_IMPORT(int) PyObject_AsFileDescriptor(PyObject *);
 
 /* The default encoding used by the platform file system APIs
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index de1a93c..a60d3ef 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -1634,7 +1634,7 @@
 }
 
 int
-PyFile_WriteString(char *s, PyObject *f)
+PyFile_WriteString(const char *s, PyObject *f)
 {
 	if (f == NULL) {
 		/* Should be caused by a pre-existing error */