diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
index ed2d4f80ff2..18fb99ce669 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
@@ -242,6 +242,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.SUPERTYPE_NOT_A_CLASS_OR_INTERFACE) { firDiagnostic ->
+        SupertypeNotAClassOrInterfaceImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.CONSTRUCTOR_IN_OBJECT) { firDiagnostic ->
         ConstructorInObjectImpl(
             firDiagnostic as FirPsiDiagnostic<*>,
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
index 59e6aea5ba1..057409db49a 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
@@ -178,6 +178,11 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = SealedSupertypeInLocalClass::class
     }
 
+    abstract class SupertypeNotAClassOrInterface : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = SupertypeNotAClassOrInterface::class
+        abstract val reason: String
+    }
+
     abstract class ConstructorInObject : KtFirDiagnostic<KtDeclaration>() {
         override val diagnosticClass get() = ConstructorInObject::class
     }
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
index 18d7cad5c93..2a6bd32af2d 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
@@ -278,6 +278,14 @@ internal class SealedSupertypeInLocalClassImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class SupertypeNotAClassOrInterfaceImpl(
+    override val reason: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.SupertypeNotAClassOrInterface(), KtAbstractFirDiagnostic<KtElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class ConstructorInObjectImpl(
     firDiagnostic: FirPsiDiagnostic<*>,
     override val token: ValidityToken,