diff --git a/src/main/java/com/taskadapter/redmineapi/internal/RedmineJSONBuilder.java b/src/main/java/com/taskadapter/redmineapi/internal/RedmineJSONBuilder.java
index 6d9458a3..01390524 100644
--- a/src/main/java/com/taskadapter/redmineapi/internal/RedmineJSONBuilder.java
+++ b/src/main/java/com/taskadapter/redmineapi/internal/RedmineJSONBuilder.java
@@ -239,6 +239,7 @@ public static void writeProject(Project project, final JSONWriter writer)
 		JsonOutput.addIfNotNull(writer, "homepage", project.getHomepage());
 		addIfNotNullFull(writer, "created_on", project.getCreatedOn());
 		addIfNotNullFull(writer, "updated_on", project.getUpdatedOn());
+		writeCustomFields(writer, project.getCustomFields());
 		JsonOutput.addIfNotNull(writer, "parent_id", project.getParentId());
                 JsonOutput.addIfNotNull(writer, "is_public", project.getProjectPublic());
 		JsonOutput.addArrayIfNotNull(writer, "trackers", project.getTrackers(),