diff --git a/docs/06_custom_tags.md b/docs/06_custom_tags.md
index 9b0d81ca..0ca17d2c 100644
--- a/docs/06_custom_tags.md
+++ b/docs/06_custom_tags.md
@@ -73,12 +73,13 @@ const regexp = {
   tag: '!re',
   resolve(str) {
     const match = str.match(/^\/([\s\S]+)\/([gimuy]*)$/)
+    if (!match) throw new Error('Invalid !re value')
     return new RegExp(match[1], match[2])
   }
 }
 
 const sharedSymbol = {
-  identify: value => value.constructor === Symbol,
+  identify: value => value?.constructor === Symbol,
   tag: '!symbol/shared',
   resolve: str => Symbol.for(str),
   stringify(item, ctx, onComment, onChompKeep) {
diff --git a/src/schema/types.ts b/src/schema/types.ts
index 9ee2368a..b20be459 100644
--- a/src/schema/types.ts
+++ b/src/schema/types.ts
@@ -18,7 +18,7 @@ interface TagBase {
    * an explicit tag. For most cases, it's unlikely that you'll actually want to
    * use this, even if you first think you do.
    */
-  default: boolean
+  default?: boolean
 
   /**
    * If a tag has multiple forms that should be parsed and/or stringified
diff --git a/src/stringify/stringifyString.ts b/src/stringify/stringifyString.ts
index c425a8b9..c71a848d 100644
--- a/src/stringify/stringifyString.ts
+++ b/src/stringify/stringifyString.ts
@@ -318,7 +318,7 @@ function plainString(
 }
 
 export function stringifyString(
-  item: Scalar,
+  item: Scalar | StringifyScalar,
   ctx: StringifyContext,
   onComment?: () => void,
   onChompKeep?: () => void
@@ -336,7 +336,7 @@ export function stringifyString(
       type = Scalar.QUOTE_DOUBLE
   }
 
-  const _stringify = (_type: Scalar.Type | undefined) => {
+  const _stringify = (_type: string | undefined) => {
     switch (_type) {
       case Scalar.BLOCK_FOLDED:
       case Scalar.BLOCK_LITERAL: