diff --git a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
index 7cbe7640626..8bf236d45ad 100644
--- a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
+++ b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java
@@ -22200,6 +22200,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
         runTest("compiler/testData/diagnostics/tests/j+k/rawOverride.kt");
       }
 
+      @Test
+      @TestMetadata("rawTypeCrash.kt")
+      public void testRawTypeCrash() {
+        runTest("compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt");
+      }
+
       @Test
       @TestMetadata("rawTypeScope.kt")
       public void testRawTypeScope() {
diff --git a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
index 219e27269a4..d67338c3110 100644
--- a/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
+++ b/analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java
@@ -22200,6 +22200,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
         runTest("compiler/testData/diagnostics/tests/j+k/rawOverride.kt");
       }
 
+      @Test
+      @TestMetadata("rawTypeCrash.kt")
+      public void testRawTypeCrash() {
+        runTest("compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt");
+      }
+
       @Test
       @TestMetadata("rawTypeScope.kt")
       public void testRawTypeScope() {
diff --git a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
index fe320f6bb9b..c3fe05bf190 100644
--- a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java
@@ -22194,6 +22194,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
         runTest("compiler/testData/diagnostics/tests/j+k/rawOverride.kt");
       }
 
+      @Test
+      @TestMetadata("rawTypeCrash.kt")
+      public void testRawTypeCrash() {
+        runTest("compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt");
+      }
+
       @Test
       @TestMetadata("rawTypeScope.kt")
       public void testRawTypeScope() {
diff --git a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
index 1478765c1c1..5ebeadaa289 100644
--- a/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java
@@ -22200,6 +22200,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
         runTest("compiler/testData/diagnostics/tests/j+k/rawOverride.kt");
       }
 
+      @Test
+      @TestMetadata("rawTypeCrash.kt")
+      public void testRawTypeCrash() {
+        runTest("compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt");
+      }
+
       @Test
       @TestMetadata("rawTypeScope.kt")
       public void testRawTypeScope() {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CreateFreshTypeVariableSubstitutorStage.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CreateFreshTypeVariableSubstitutorStage.kt
index fd15cce144d..9fabf627ea4 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CreateFreshTypeVariableSubstitutorStage.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CreateFreshTypeVariableSubstitutorStage.kt
@@ -141,8 +141,25 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
             containingDeclarationSymbol !is FirSyntheticFunctionSymbol &&
             typeParameter.shouldBeFlexible(session.typeContext)
         ) {
-            val notNullType = type.withNullability(ConeNullability.NOT_NULL, session.typeContext) as ConeSimpleKotlinType
-            ConeFlexibleType(notNullType, notNullType.withNullability(ConeNullability.NULLABLE, session.typeContext))
+            when (type) {
+                is ConeSimpleKotlinType -> ConeFlexibleType(
+                    type.withNullability(ConeNullability.NOT_NULL, session.typeContext),
+                    type.withNullability(ConeNullability.NULLABLE, session.typeContext)
+                )
+                /*
+                 * ConeFlexibleTypes have to be handled here
+                 * at least because MapTypeArguments special-cases ConeRawTypes without explicit arguments (KT-54666)
+                 * which allows them to get past the NoExplicitArguments optimization
+                 * in CreateFreshTypeVariableSubstitutorStage.check
+                 *
+                 * (it might be safe to just return the same flexible type without explicitly enforcing flexibility,
+                 * but better safe than sorry when dealing with raw types)
+                 */
+                is ConeFlexibleType -> ConeFlexibleType(
+                    type.lowerBound.withNullability(ConeNullability.NOT_NULL, session.typeContext),
+                    type.upperBound.withNullability(ConeNullability.NULLABLE, session.typeContext)
+                )
+            }
         } else {
             type
         }
diff --git a/compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt b/compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt
new file mode 100644
index 00000000000..bc3a1bee481
--- /dev/null
+++ b/compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt
@@ -0,0 +1,25 @@
+// FIR_IDENTICAL
+// ISSUE: KT-66552
+
+// FILE: JsStubElementType.java
+public interface JsStubElementType<E, S extends JsStubElement<E>> {}
+
+// FILE: JsStubElement.java
+public interface JsStubElement<T> {
+    <E, S extends JsStubElement<E>> void findChildStubByType(JsStubElementType<E, S> childStubType);
+}
+
+// FILE: FromJava.java
+public interface FromJava {
+    JsStubElement RAW_TYPED_STUB = null;
+}
+
+// FILE: Kotlin.kt
+interface JsStubElementSubType: JsStubElement<Any>
+
+fun test(childStubType: JsStubElementType<Any, JsStubElement<Any>>) {
+    val rawTypedStub = FromJava.RAW_TYPED_STUB
+    if (rawTypedStub is JsStubElementSubType) {
+        rawTypedStub.findChildStubByType(childStubType)
+    }
+}
\ No newline at end of file
diff --git a/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java b/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
index 671159c0002..1b6cb00d1bf 100644
--- a/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
+++ b/compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java
@@ -22200,6 +22200,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
         runTest("compiler/testData/diagnostics/tests/j+k/rawOverride.kt");
       }
 
+      @Test
+      @TestMetadata("rawTypeCrash.kt")
+      public void testRawTypeCrash() {
+        runTest("compiler/testData/diagnostics/tests/j+k/rawTypeCrash.kt");
+      }
+
       @Test
       @TestMetadata("rawTypeScope.kt")
       public void testRawTypeScope() {