diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt
index 2a666e2fb81..f02ec8834bf 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt
@@ -148,7 +148,7 @@ object FirDiagnosticRenderers {
     private const val WHEN_MISSING_LIMIT = 7
 
     val WHEN_MISSING_CASES = Renderer { missingCases: List<WhenMissingCase> ->
-        if (missingCases.firstOrNull() == WhenMissingCase.Unknown) {
+        if (missingCases.singleOrNull() == WhenMissingCase.Unknown) {
             "'else' branch"
         } else {
             val list = missingCases.joinToString(", ", limit = WHEN_MISSING_LIMIT) { "'$it'" }