diff --git a/fx/include/fx/string.h b/fx/include/fx/string.h
index 1873513..8b6ed02 100644
--- a/fx/include/fx/string.h
+++ b/fx/include/fx/string.h
@@ -95,7 +95,7 @@ fx_string_insert_cstrf(fx_string *dest, size_t at, const char *format, ...);
 FX_API void fx_string_clear(fx_string *str);
 
 FX_API fx_iterator *fx_string_tokenise(
-	fx_string *str,
+	const fx_string *str,
 	const char *delims[],
 	size_t nr_delims,
 	fx_string_tokenise_flags flags);
diff --git a/fx/string.c b/fx/string.c
index 0e6d5db..83d6d9e 100644
--- a/fx/string.c
+++ b/fx/string.c
@@ -56,7 +56,7 @@ struct fx_string_p {
 struct fx_string_iterator_p {
 	int _m, _f;
 	fx_string *_tmp;
-	struct fx_string_p *_s_p, *_tmp_p;
+	const struct fx_string_p *_s_p, *_tmp_p;
 
 	const char **_d;
 	size_t _nd, _ds;
@@ -1023,7 +1023,7 @@ static enum fx_status find_next_token(struct fx_string_iterator_p *it)
 }
 
 static fx_iterator *string_tokenise(
-	struct fx_string_p *str,
+	const struct fx_string_p *str,
 	const char *delims[],
 	size_t nr_delims,
 	fx_string_tokenise_flags flags)
@@ -1390,7 +1390,7 @@ void fx_string_clear(fx_string *str)
 }
 
 fx_iterator *fx_string_tokenise(
-	fx_string *str,
+	const fx_string *str,
 	const char *delims[],
 	size_t nr_delims,
 	fx_string_tokenise_flags flags)