diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/ConeDiagnostic.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/ConeDiagnostic.kt
index 75cae4bb49e..48ff0bf9fde 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/ConeDiagnostic.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/ConeDiagnostic.kt
@@ -16,21 +16,25 @@ class ConeDiagnostic(
 )
 
 inline fun <reified E : PsiElement> DiagnosticFactory0<E>.onSource(source: FirSourceElement): ConeDiagnostic? {
-    val psi = source.psi as? E ?: return null
+    val psi = source.psi as? E
+        ?: throw IllegalArgumentException("Incompatible PSI: expected ${E::class}, actual ${source.psi?.let { it::class }}")
     return ConeDiagnostic(this.on(psi), source)
 }
 
 inline fun <reified E : PsiElement, A> DiagnosticFactory1<E, A>.onSource(source: FirSourceElement, a: A): ConeDiagnostic? {
-    val psi = source.psi as? E ?: return null
+    val psi = source.psi as? E
+        ?: throw IllegalArgumentException("Incompatible PSI: expected ${E::class}, actual ${source.psi?.let { it::class }}")
     return ConeDiagnostic(this.on(psi, a), source)
 }
 
 inline fun <reified E : PsiElement, A, B> DiagnosticFactory2<E, A, B>.onSource(source: FirSourceElement, a: A, b: B): ConeDiagnostic? {
-    val psi = source.psi as? E ?: return null
+    val psi = source.psi as? E
+        ?: throw IllegalArgumentException("Incompatible PSI: expected ${E::class}, actual ${source.psi?.let { it::class }}")
     return ConeDiagnostic(this.on(psi, a, b), source)
 }
 
 inline fun <reified E : PsiElement, A, B, C> DiagnosticFactory3<E, A, B, C>.onSource(source: FirSourceElement, a: A, b: B, c: C): ConeDiagnostic? {
-    val psi = source.psi as? E ?: return null
+    val psi = source.psi as? E
+        ?: throw IllegalArgumentException("Incompatible PSI: expected ${E::class}, actual ${source.psi?.let { it::class }}")
     return ConeDiagnostic(this.on(psi, a, b, c), source)
 }