diff --git a/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java b/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
index 62073456fc82..c4c5ec6c53bd 100644
--- a/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
+++ b/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
@@ -221,7 +221,10 @@ public final class PsiSubstitutorImpl implements PsiSubstitutor {
       while (true) {
         final PsiTypeParameter[] params = resolve.getTypeParameters();
         for (final PsiTypeParameter param : params) {
-          final PsiType original = originalSubstitutor.substitute(param);
+          PsiType original = originalSubstitutor.substitute(param);
+          if (original != null && original.getNullability().equals(TypeNullability.UNKNOWN)) {
+            original = original.withNullability(TypeNullability.ofTypeParameter(param));
+          }
           PsiType mapping = original == null ? null : original.accept(this);
           substMap = substMap.with(param, mapping);
         }
diff --git a/java/java-tests/testData/inspection/dataFlow/fixture/DelegateClassTypeParameter.java b/java/java-tests/testData/inspection/dataFlow/fixture/DelegateClassTypeParameter.java
new file mode 100644
index 000000000000..e907877552b3
--- /dev/null
+++ b/java/java-tests/testData/inspection/dataFlow/fixture/DelegateClassTypeParameter.java
@@ -0,0 +1,63 @@
+package org.example;
+
+import org.jspecify.annotations.*;
+
+@NullMarked
+class CustomOptionalTest2 {
+  @Nullable
+  public String a;
+  public String @Nullable [] a3;
+
+  CustomOptional2<String> returnNotNullableDelegate() {
+    return Delegate2.ofNullable(a);
+  }
+
+  CustomOptional2<String> returnNotNullableDelegate2() {
+    return Delegate2.ofNullable2(a);
+  }
+
+  CustomOptional2<String[]> returnNotNullableDelegate3() {
+    return Delegate2.ofNullable3(a3);
+  }
+
+  CustomOptional2<?> returnNotNullableDelegate4() {
+    return Delegate2.ofNullable4(a3);
+  }
+
+  CustomOptional2<?> returnNotNullableDelegate5() {
+    return Delegate2.ofNullable4(a3);
+  }
+}
+
+class Delegate2 {
+  static <T> CustomOptional2<T> ofNullable(@Nullable T t) {
+    return CustomOptional2.ofNullable(t);
+  }
+
+  static CustomOptional2<String> ofNullable2(@Nullable String t) {
+    return CustomOptional2.ofNullable(t);
+  }
+
+  static CustomOptional2<String[]> ofNullable3(@Nullable String @Nullable [] t) {
+    return CustomOptional2.ofNullable(t);
+  }
+
+  static <T extends Object> CustomOptional2<? extends T> ofNullable4(T t) {
+    return CustomOptional2.ofNullable(t);
+  }
+
+  static CustomOptional2<?> ofNullable5(Object t) {
+    return CustomOptional2.ofNullable(t);
+  }
+}
+
+class CustomOptional2<T extends @NotNull Object> {
+  public CustomOptional2(T s) {
+
+  }
+
+  public static <T> CustomOptional2<@NotNull T> ofNullable(@Nullable T value) {
+    return value == null ? (CustomOptional2<T>) new CustomOptional2<>("")
+                         : new CustomOptional2<>(value);
+  }
+}
\ No newline at end of file
diff --git a/java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java b/java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java
index 036fab92629b..f5ec0d64024f 100644
--- a/java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java
+++ b/java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java
@@ -256,6 +256,12 @@ public class DataFlowInspection8Test extends DataFlowInspectionTestCase {
   public void testFlatMapSideEffect() { doTest(); }
   public void testOptionalValueTracking() { doTest(); }
   public void testOptionalAsQualifier() { doTest(); }
+  public void testDelegateClassTypeParameter() {
+    addJSpecifyNullMarked(myFixture);
+    setupTypeUseAnnotations("org.jspecify.annotations", myFixture);
+    doTest();
+  }
+
   public void testClearZeroesSize() { doTest(); }
   public void testLambdaInlineReassignReturnWithDeeperEquality() { doTest(); }