diff --git a/core/server/models/base/index.js b/core/server/models/base/index.js
index 018f26a49d..60c1b2612c 100644
--- a/core/server/models/base/index.js
+++ b/core/server/models/base/index.js
@@ -311,13 +311,13 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
     onCreating: function onCreating(model, attr, options) {
         if (schema.tables[this.tableName].hasOwnProperty('created_by')) {
             if (!options.importing || (options.importing && !this.get('created_by'))) {
-                this.set('created_by', this.contextUser(options));
+                this.set('created_by', String(this.contextUser(options)));
             }
         }
 
         if (schema.tables[this.tableName].hasOwnProperty('updated_by')) {
             if (!options.importing) {
-                this.set('updated_by', this.contextUser(options));
+                this.set('updated_by', String(this.contextUser(options)));
             }
         }
 
@@ -377,7 +377,7 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
 
         if (schema.tables[this.tableName].hasOwnProperty('updated_by')) {
             if (!options.importing && !options.migrating) {
-                this.set('updated_by', this.contextUser(options));
+                this.set('updated_by', String(this.contextUser(options)));
             }
         }
 
@@ -390,7 +390,7 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
 
             if (schema.tables[this.tableName].hasOwnProperty('created_by')) {
                 if (model.hasChanged('created_by')) {
-                    model.set('created_by', this.previous('created_by'));
+                    model.set('created_by', String(this.previous('created_by')));
                 }
             }
         }
diff --git a/core/server/models/post.js b/core/server/models/post.js
index bff7a4d803..aba1ef137b 100644
--- a/core/server/models/post.js
+++ b/core/server/models/post.js
@@ -388,12 +388,12 @@ Post = ghostBookshelf.Model.extend({
         if (newStatus === 'published' && this.hasChanged('status')) {
             // unless published_by is set and we're importing, set published_by to contextUser
             if (!(this.get('published_by') && options.importing)) {
-                this.set('published_by', this.contextUser(options));
+                this.set('published_by', String(this.contextUser(options)));
             }
         } else {
             // In any other case (except import), `published_by` should not be changed
             if (this.hasChanged('published_by') && !options.importing) {
-                this.set('published_by', this.previous('published_by') || null);
+                this.set('published_by', this.previous('published_by') ? String(this.previous('published_by')) : null);
             }
         }