diff --git a/src/commonMain/kotlin/com/charleskorn/kaml/YamlInput.kt b/src/commonMain/kotlin/com/charleskorn/kaml/YamlInput.kt
index 5d2a51d1..b59f8ddb 100644
--- a/src/commonMain/kotlin/com/charleskorn/kaml/YamlInput.kt
+++ b/src/commonMain/kotlin/com/charleskorn/kaml/YamlInput.kt
@@ -30,6 +30,7 @@ import kotlinx.serialization.descriptors.StructureKind
 import kotlinx.serialization.descriptors.elementNames
 import kotlinx.serialization.encoding.AbstractDecoder
 import kotlinx.serialization.encoding.CompositeDecoder
+import kotlinx.serialization.encoding.CompositeDecoder.Companion.DECODE_DONE
 import kotlinx.serialization.encoding.CompositeDecoder.Companion.UNKNOWN_NAME
 import kotlinx.serialization.modules.SerializersModule
 import kotlinx.serialization.modules.SerializersModuleCollector
@@ -168,7 +169,7 @@ private class YamlNullInput(val nullValue: YamlNode, context: SerializersModule,
     override fun getCurrentLocation(): Location = nullValue.location
     override fun getCurrentPath(): YamlPath = nullValue.path
 
-    override fun decodeElementIndex(descriptor: SerialDescriptor): Int = 0
+    override fun decodeElementIndex(descriptor: SerialDescriptor): Int = DECODE_DONE
 }
 
 @OptIn(ExperimentalSerializationApi::class)
diff --git a/src/commonTest/kotlin/com/charleskorn/kaml/YamlReadingTest.kt b/src/commonTest/kotlin/com/charleskorn/kaml/YamlReadingTest.kt
index e8e1beba..9adcff53 100644
--- a/src/commonTest/kotlin/com/charleskorn/kaml/YamlReadingTest.kt
+++ b/src/commonTest/kotlin/com/charleskorn/kaml/YamlReadingTest.kt
@@ -1193,6 +1193,25 @@ object YamlReadingTest : Spek({
                     }
                 }
 
+                // See https://github.com/charleskorn/kaml/issues/179.
+                context("given some input where a tag is provided but no value is provided") {
+                    val input = """
+                        !<sealedString>
+                    """.trimIndent()
+
+                    context("parsing that input") {
+                        it("throws an appropriate exception") {
+                            expect({ polymorphicYaml.decodeFromString(TestSealedStructure.serializer(), input) }).toThrow<MissingRequiredPropertyException> {
+                                message { toBe("Property 'value' is required but it is missing.") }
+                                line { toBe(1) }
+                                column { toBe(1) }
+                                propertyName { toBe("value") }
+                                path { toBe(YamlPath.root) }
+                            }
+                        }
+                    }
+                }
+
                 context("given some input where the value is a literal") {
                     val input = """
                         !<simpleString> "asdfg"