diff --git a/json-schema.c b/json-schema.c
index 4a9048a..e0f55c0 100644
--- a/json-schema.c
+++ b/json-schema.c
@@ -25,7 +25,7 @@
 int validate_integer(const char *field_name, json_object *schema,
 		     json_object *object, char *error_message);
 int validate_string(const char *field_name, json_object *schema,
-		    json_object *object, char *error_message);
+		    json_object *object, const char *error_message);
 int validate_object(const char *field_name, json_object *schema,
 		    json_object *object, char *error_message);
 int validate_array(const char *field_name, json_object *schema,
@@ -284,7 +284,7 @@
 
 //Validates a single string value according to the given specification.
 int validate_string(const char *field_name, json_object *schema,
-		    json_object *object, char *error_message)
+		    json_object *object, const char *error_message)
 {
 	//todo: if there is a "pattern" field, verify the string with RegEx.
 	(void)field_name;