diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/utils/ArrayMap.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/utils/ArrayMap.kt
index 0a685e62688..6d2070d5b61 100644
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/utils/ArrayMap.kt
+++ b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/utils/ArrayMap.kt
@@ -79,7 +79,7 @@ internal class ArrayMapImpl<T : Any> : ArrayMap<T>() {
 
     private var data = arrayOfNulls<Any>(DEFAULT_SIZE)
     private fun ensureCapacity(index: Int) {
-        if (data.size < index) {
+        if (data.size <= index) {
             data = data.copyOf(data.size * INCREASE_K)
         }
     }