diff --git a/db/create.go b/db/create.go
index fc1ab99..9032770 100644
--- a/db/create.go
+++ b/db/create.go
@@ -177,7 +177,11 @@ func (c *Column) String() (string, error) {
 
 	if c.Default.Set {
 		str.WriteString(" DEFAULT ")
-		str.WriteString(c.Default.Value)
+		val := c.Default.Value
+		if val == "" {
+			val = "''"
+		}
+		str.WriteString(val)
 	}
 
 	if c.PrimaryKey {
@@ -250,4 +254,3 @@ func (b *CreateTableSqlBuilder) ToSQL() (string, error) {
 
 	return str.String(), nil
 }
-
diff --git a/migrations/v5.go b/migrations/v5.go
index dc404e7..88ddd28 100644
--- a/migrations/v5.go
+++ b/migrations/v5.go
@@ -20,35 +20,35 @@ func oauthSlack(db *datastore) error {
 					Column(
 						"provider",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 24})),
+						wf_db.OptionalInt{Set: true, Value: 24}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_client_states").
 				AddColumn(dialect.
 					Column(
 						"client_id",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 128})),
+						wf_db.OptionalInt{Set: true, Value: 128}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"provider",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 24})),
+						wf_db.OptionalInt{Set: true, Value: 24}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"client_id",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 128})),
+						wf_db.OptionalInt{Set: true, Value: 128}).SetDefault("")),
 			dialect.
 				AlterTable("oauth_users").
 				AddColumn(dialect.
 					Column(
 						"access_token",
 						wf_db.ColumnTypeVarChar,
-						wf_db.OptionalInt{Set: true, Value: 512,})),
+						wf_db.OptionalInt{Set: true, Value: 512}).SetDefault("")),
 			dialect.CreateUniqueIndex("oauth_users", "oauth_users", "user_id", "provider", "client_id"),
 		}
 
@@ -63,6 +63,7 @@ func oauthSlack(db *datastore) error {
 							wf_db.ColumnTypeVarChar,
 							wf_db.OptionalInt{Set: true, Value: 128})))
 		}
+
 		for _, builder := range builders {
 			query, err := builder.ToSQL()
 			if err != nil {