diff --git a/src/main/kotlin/ru/dbotthepony/mc/otm/core/collect/ConditionalSet.kt b/src/main/kotlin/ru/dbotthepony/mc/otm/core/collect/ConditionalSet.kt
index 9596f15e9..7b70a3f0a 100644
--- a/src/main/kotlin/ru/dbotthepony/mc/otm/core/collect/ConditionalSet.kt
+++ b/src/main/kotlin/ru/dbotthepony/mc/otm/core/collect/ConditionalSet.kt
@@ -30,6 +30,10 @@ class ConditionalSet<E : Any> : Set<E> {
 		return backing.entries.iterator().filter { it.value.asBoolean }.map { it.key }
 	}
 
+	fun actuallyContains(element: E): Boolean {
+		return element in backing
+	}
+
 	fun add(element: E, condition: BooleanSupplier): Boolean {
 		if (element in backing) return false
 		backing[element] = condition
diff --git a/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt b/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
index 702258b2e..0e4260970 100644
--- a/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
+++ b/src/main/kotlin/ru/dbotthepony/mc/otm/menu/MatteryMenu.kt
@@ -440,7 +440,7 @@ abstract class MatteryMenu @JvmOverloads protected constructor(
 	 * [condition] allows to specify when slot is invisible on GUI (hence being ignored by quick move)
 	 */
 	protected fun <T : Slot> addStorageSlot(slot: T, addMapping: Boolean = true, prepend: Boolean = false, condition: BooleanSupplier = BooleanSupplier { true }): T {
-		if (slot !in externalSlots) {
+		if (!externalSlots.actuallyContains(slot)) {
 			addSlot(slot)
 
 			if (prepend)