diff --git a/include/rjp.h b/include/rjp.h
index c73acaa..2f5f95b 100644
--- a/include/rjp.h
+++ b/include/rjp.h
@@ -130,7 +130,7 @@ RJP_value* rjp_parse(const char* str, int flags, RJP_parse_error* err);
 RJP_value* rjp_parse_cback(int flags, RJP_parse_callback* cbacks, RJP_parse_error* err);
 char* rjp_to_json(const RJP_value* root, int pretty);
 
-char* rjp_parse_error_to_string(RJP_parse_error* err);
+char* rjp_parse_error_to_string(const RJP_parse_error* err);
 void rjp_delete_parse_error(RJP_parse_error* err);
 
 RJP_value* rjp_new_null(void);
diff --git a/src/rjp_parse.c b/src/rjp_parse.c
index 940c249..54af75f 100644
--- a/src/rjp_parse.c
+++ b/src/rjp_parse.c
@@ -317,8 +317,8 @@ static int irjp_parse_cback(RJP_parse_state* state, RJP_parse_callback* cback){
 	return irjp_handle_final_parse_token(state, cat);
 }
 
-char* rjp_parse_error_to_string(RJP_parse_error* err){
-	RJP_parse_state* state = (RJP_parse_state*)err->parsestate;
+char* rjp_parse_error_to_string(const RJP_parse_error* err){
+	const RJP_parse_state* state = (const RJP_parse_state*)err->parsestate;
 	RJP_parse_status status = err->errcode;
 	char* buffer = NULL;
 	const char* format = NULL;