diff --git a/build-common/src/org/jetbrains/kotlin/incremental/IncrementalJvmCache.kt b/build-common/src/org/jetbrains/kotlin/incremental/IncrementalJvmCache.kt
index 35884af0d79..73fd16ca986 100644
--- a/build-common/src/org/jetbrains/kotlin/incremental/IncrementalJvmCache.kt
+++ b/build-common/src/org/jetbrains/kotlin/incremental/IncrementalJvmCache.kt
@@ -429,6 +429,8 @@ open class IncrementalJvmCache(
 
     private inner class MultifileClassFacadeMap(storageFile: File) :
         BasicStringMap<Collection<String>>(storageFile, StringCollectionExternalizer) {
+
+        @Synchronized
         operator fun set(className: JvmClassName, partNames: Collection<String>) {
             storage[className.internalName] = partNames
         }
@@ -439,6 +441,7 @@ open class IncrementalJvmCache(
         operator fun contains(className: JvmClassName): Boolean =
             className.internalName in storage
 
+        @Synchronized
         fun remove(className: JvmClassName) {
             storage.remove(className.internalName)
         }
@@ -448,6 +451,8 @@ open class IncrementalJvmCache(
 
     private inner class MultifileClassPartMap(storageFile: File) :
         BasicStringMap<String>(storageFile, EnumeratorStringDescriptor.INSTANCE) {
+
+        @Synchronized
         fun set(partName: String, facadeName: String) {
             storage[partName] = facadeName
         }
@@ -455,6 +460,7 @@ open class IncrementalJvmCache(
         fun get(partName: JvmClassName): String? =
             storage[partName.internalName]
 
+        @Synchronized
         fun remove(className: JvmClassName) {
             storage.remove(className.internalName)
         }