diff --git a/src/calibre/utils/icu_calibre_utils.h b/src/calibre/utils/icu_calibre_utils.h
index 283a7f575d..d80a62da97 100644
--- a/src/calibre/utils/icu_calibre_utils.h
+++ b/src/calibre/utils/icu_calibre_utils.h
@@ -133,7 +133,7 @@ static UChar32* python_to_icu32(PyObject *obj, int32_t *osz) {
 #endif
 
 #ifndef NO_ICU_TO_PYTHON
-static PyObject* icu_to_python(UChar *src, int32_t sz) {
-    return PyUnicode_DecodeUTF16((char*) src, sz * sizeof(UChar), "replace", NULL);
+static PyObject* icu_to_python(const UChar *src, int32_t sz) {
+    return PyUnicode_DecodeUTF16((const char*) src, sz * sizeof(UChar), "replace", NULL);
 }
 #endif