diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirAbstractPhaseTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirAbstractPhaseTransformer.kt
index 58b8ec29571..06c8cf7a662 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirAbstractPhaseTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirAbstractPhaseTransformer.kt
@@ -34,6 +34,7 @@ abstract class FirAbstractPhaseTransformer<D>(
         }
 
     override fun transformFile(file: FirFile, data: D): CompositeTransformResult<FirFile> {
+        checkSessionConsistency(file)
         file.replaceResolvePhase(transformerPhase)
 
         @Suppress("UNCHECKED_CAST")
@@ -45,6 +46,12 @@ abstract class FirAbstractPhaseTransformer<D>(
 
         return super.transformDeclaration(declaration, data)
     }
+
+    protected fun checkSessionConsistency(file: FirFile) {
+        assert(session === file.session) {
+            "File ${file.name} and transformer ${this::class} have inconsistent sessions"
+        }
+    }
 }
 
 fun FirFile.runResolve(toPhase: FirResolvePhase, fromPhase: FirResolvePhase = FirResolvePhase.RAW_FIR) {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirImportResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirImportResolveTransformer.kt
index 9ebe7b30e7b..868469a2a04 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirImportResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirImportResolveTransformer.kt
@@ -19,27 +19,21 @@ import org.jetbrains.kotlin.fir.visitors.compose
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 
-open class FirImportResolveTransformer protected constructor(phase: FirResolvePhase) : FirAbstractTreeTransformer<Nothing?>(phase) {
-    constructor() : this(FirResolvePhase.IMPORTS)
-
+open class FirImportResolveTransformer protected constructor(
+    final override val session: FirSession,
+    phase: FirResolvePhase
+) : FirAbstractTreeTransformer<Nothing?>(phase) {
     override fun <E : FirElement> transformElement(element: E, data: Nothing?): CompositeTransformResult<E> {
         return element.compose()
     }
 
-    private lateinit var symbolProvider: FirSymbolProvider
+    constructor(session: FirSession) : this(session, FirResolvePhase.IMPORTS)
 
-    final override lateinit var session: FirSession
-
-    constructor(session: FirSession) : this() {
-        this.session = session
-        // TODO: clarify this
-        symbolProvider = session.firSymbolProvider
-    }
+    private val symbolProvider: FirSymbolProvider = session.firSymbolProvider
 
     override fun transformFile(file: FirFile, data: Nothing?): CompositeTransformResult<FirFile> {
+        checkSessionConsistency(file)
         file.replaceResolvePhase(transformerPhase)
-        session = file.session
-        symbolProvider = file.session.firSymbolProvider
         return file.also { it.transformChildren(this, null) }.compose()
     }
 
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ResolvePhaseUtils.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ResolvePhaseUtils.kt
index 7856dfbb7e4..7a3b6b51dfa 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ResolvePhaseUtils.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ResolvePhaseUtils.kt
@@ -23,7 +23,7 @@ fun FirResolvePhase.createTransformerByPhase(session: FirSession, scopeSession:
         RAW_FIR -> throw AssertionError("Raw FIR building phase does not have a transformer")
         ANNOTATIONS_FOR_PLUGINS -> FirPluginAnnotationsResolveTransformer(scopeSession)
         FIRST_PLUGIN_GENERATION -> FirFirstGenerationTransformer()
-        IMPORTS -> FirImportResolveTransformer()
+        IMPORTS -> FirImportResolveTransformer(session)
         SUPER_TYPES -> FirSupertypeResolverTransformer(scopeSession)
         SEALED_CLASS_INHERITORS -> FirSealedClassInheritorsTransformer()
         TYPES -> FirTypeResolveTransformerAdapter(scopeSession)