diff --git a/src/parse/parser.ts b/src/parse/parser.ts
index af09b527..3c26577e 100644
--- a/src/parse/parser.ts
+++ b/src/parse/parser.ts
@@ -529,9 +529,9 @@ export class Parser {
     }
 
     if (this.indent >= map.indent) {
+      const atMapIndent = !this.onKeyLine && this.indent === map.indent
       const atNextItem =
-        !this.onKeyLine &&
-        this.indent === map.indent &&
+        atMapIndent &&
         (it.sep || it.explicitKey) &&
         this.type !== 'seq-item-ind'
 
@@ -673,7 +673,7 @@ export class Parser {
         default: {
           const bv = this.startBlockValue(map)
           if (bv) {
-            if (atNextItem && bv.type !== 'block-seq' && it.explicitKey) {
+            if (atMapIndent && bv.type !== 'block-seq') {
               map.items.push({ start })
             }
             this.stack.push(bv)
diff --git a/tests/doc/parse.ts b/tests/doc/parse.ts
index a6af6764..154db795 100644
--- a/tests/doc/parse.ts
+++ b/tests/doc/parse.ts
@@ -422,6 +422,16 @@ describe('odd indentations', () => {
     const doc = YAML.parseDocument<YAML.YAMLMap, false>('?\n!!null')
     expect(doc.errors).not.toHaveLength(0)
   })
+
+  test('unindented block scalar header in mapping value (#553)', () => {
+    const doc = YAML.parseDocument<YAML.YAMLMap, false>('a:\n|\n x')
+    expect(doc.errors).not.toHaveLength(0)
+  })
+
+  test('unindented flow collection in mapping value', () => {
+    const doc = YAML.parseDocument<YAML.YAMLMap, false>('a:\n{x}')
+    expect(doc.errors).not.toHaveLength(0)
+  })
 })
 
 describe('Excessive entity expansion attacks', () => {