diff --git a/plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt b/plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt
index 6d16304bb53..eab61372512 100644
--- a/plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt
+++ b/plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt
@@ -455,7 +455,7 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
             val serializerSymbol = customSerializerType?.toRegularClassSymbol(session)
             val propertySymbol = property.propertySymbol
             val typeRef = propertySymbol.resolvedReturnTypeRef
-            val propertyType = typeRef.coneType
+            val propertyType = typeRef.coneType.fullyExpandedType(session)
             val source = typeRef.source ?: propertySymbol.source
             if (customSerializerType != null && serializerSymbol != null) {
                 // Do not account for @Polymorphic and @Contextual, as they are serializers for T: Any
@@ -498,7 +498,7 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
     private fun checkTypeArguments(type: ConeKotlinType, source: KtSourceElement?, reporter: DiagnosticReporter) {
         for (typeArgument in type.typeArguments) {
             checkType(
-                typeArgument.type ?: continue,
+                typeArgument.type?.fullyExpandedType(session) ?: continue,
                 source,
                 reporter
             )
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/typeAliases.kt b/plugins/kotlinx-serialization/testData/diagnostics/typeAliases.kt
new file mode 100644
index 00000000000..c35f928eb17
--- /dev/null
+++ b/plugins/kotlinx-serialization/testData/diagnostics/typeAliases.kt
@@ -0,0 +1,23 @@
+// FIR_IDENTICAL
+// FIR_DISABLE_LAZY_RESOLVE_CHECKS
+// WITH_STDLIB
+// SKIP_TXT
+
+import kotlinx.serialization.*
+
+typealias MyString = String
+typealias MyLong = Long
+
+@Serializable
+class Box<T>(val t: T)
+
+typealias MyBox<T> = Box<T>
+
+@Serializable
+class Foo(
+    val s: MyString,
+    val l: MyLong,
+    val b: Box<MyLong>,
+    val bb: MyBox<Long>,
+    val bbb: MyBox<MyLong>
+)
\ No newline at end of file
diff --git a/plugins/kotlinx-serialization/testData/diagnostics/typeAliasesCustomized.kt b/plugins/kotlinx-serialization/testData/diagnostics/typeAliasesCustomized.kt
new file mode 100644
index 00000000000..72008042343
--- /dev/null
+++ b/plugins/kotlinx-serialization/testData/diagnostics/typeAliasesCustomized.kt
@@ -0,0 +1,22 @@
+// FIR_IDENTICAL
+// FIR_DISABLE_LAZY_RESOLVE_CHECKS
+// WITH_STDLIB
+// SKIP_TXT
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+
+@Serializable(ExplicitSerializer::class)
+data class Klass(val s: String)
+
+object ExplicitSerializer : KSerializer<Klass> {
+    override val descriptor: SerialDescriptor get() = PrimitiveSerialDescriptor("klass", PrimitiveKind.STRING)
+    override fun serialize(encoder: Encoder, value: Klass) { encoder.encodeString(value.s) }
+    override fun deserialize(decoder: Decoder): Klass { return Klass(decoder.decodeString()) }
+}
+
+typealias KlassAlias = Klass
+
+@Serializable
+data class DataKlass(val k: KlassAlias)
\ No newline at end of file
diff --git a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirPsiDiagnosticTestGenerated.java b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirPsiDiagnosticTestGenerated.java
index 16e71234ffc..d6bffa4fbe1 100644
--- a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirPsiDiagnosticTestGenerated.java
+++ b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirPsiDiagnosticTestGenerated.java
@@ -151,6 +151,18 @@ public class SerializationFirPsiDiagnosticTestGenerated extends AbstractSerializ
         public void testTransients() throws Exception {
             runTest("plugins/kotlinx-serialization/testData/diagnostics/Transients.kt");
         }
+
+        @Test
+        @TestMetadata("typeAliases.kt")
+        public void testTypeAliases() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/diagnostics/typeAliases.kt");
+        }
+
+        @Test
+        @TestMetadata("typeAliasesCustomized.kt")
+        public void testTypeAliasesCustomized() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/diagnostics/typeAliasesCustomized.kt");
+        }
     }
 
     @Nested
diff --git a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java
index 445a502c366..c6fd7ef3bd9 100644
--- a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java
+++ b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java
@@ -149,4 +149,16 @@ public class SerializationPluginDiagnosticTestGenerated extends AbstractSerializ
     public void testTransients() throws Exception {
         runTest("plugins/kotlinx-serialization/testData/diagnostics/Transients.kt");
     }
+
+    @Test
+    @TestMetadata("typeAliases.kt")
+    public void testTypeAliases() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/diagnostics/typeAliases.kt");
+    }
+
+    @Test
+    @TestMetadata("typeAliasesCustomized.kt")
+    public void testTypeAliasesCustomized() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/diagnostics/typeAliasesCustomized.kt");
+    }
 }