diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/ConversionUtils.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/ConversionUtils.kt
index f9385aba0a9..b183db96278 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/ConversionUtils.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/ConversionUtils.kt
@@ -146,7 +146,8 @@ private fun FirCallableSymbol<*>.toSymbol(declarationStorage: Fir2IrDeclarationS
             val delegateSymbol = if (preferGetter) {
                 syntheticProperty.getter.delegate.symbol
             } else {
-                syntheticProperty.setter!!.delegate.symbol
+                syntheticProperty.setter?.delegate?.symbol
+                    ?: throw AssertionError("Written synthetic property must have a setter")
             }
             delegateSymbol.deepestMatchingOverriddenSymbol().toSymbol(declarationStorage, preferGetter)
         } ?: declarationStorage.getIrPropertySymbol(this)