diff --git a/CHANGES.md b/CHANGES.md
index ee531ac510..3f8f699b84 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,6 +10,8 @@ This document is intended for Spotless developers.
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `1.27.0`).
 
 ## [Unreleased]
+### Fixed
+- `ConfigurationCacheHackList` no longer throws `IllegalStateException` ("If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null") during Gradle input fingerprinting when a step's equality state is `null` (e.g. a step wrapped in `toggleOffOn()`/a fence whose state is not yet provisioned). ([#2950](https://github.com/diffplug/spotless/issues/2950))
 
 ## [4.6.2] - 2026-05-27
 ### Fixed
diff --git a/lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java b/lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java
index 15febb27ff..1343a11529 100644
--- a/lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java
+++ b/lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2025 DiffPlug
+ * Copyright 2023-2026 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -69,10 +69,11 @@ protected FormatterFunc stateToFormatter(EqualityState equalityState) throws Exc
 
 	private void writeObject(ObjectOutputStream out) throws IOException {
 		if (initializer == null) {
-			// then this instance was created by Gradle's ConfigurationCacheHackList and the following will hold true
-			if (roundtripStateInternal == null && equalityStateInternal == null) {
-				throw new IllegalStateException("If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null, and neither was");
-			}
+			// then this instance was created by Gradle's ConfigurationCacheHackList. HackClone populated
+			// exactly one of roundtripStateInternal / equalityStateInternal; it is legitimate for that value
+			// to be null (e.g. a step whose equality state is null), which still serializes deterministically.
+			// An equality-optimized clone is only ever serialized for its cache-key bytes, never rehydrated for
+			// use, so a both-null clone is safe and must not blow up Gradle's input fingerprinting (see #2950).
 		} else {
 			// this was a normal instance, which means we need to encode to roundtripStateInternal (since the initializer might not be serializable)
 			// and there's no reason to keep equalityStateInternal since we can always recompute it
diff --git a/lib/src/test/java/com/diffplug/spotless/ConfigurationCacheHackListTest.java b/lib/src/test/java/com/diffplug/spotless/ConfigurationCacheHackListTest.java
new file mode 100644
index 0000000000..1ebd4e2834
--- /dev/null
+++ b/lib/src/test/java/com/diffplug/spotless/ConfigurationCacheHackListTest.java
@@ -0,0 +1,65 @@
+/*
+ * Copyright 2024-2026 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless;
+
+import java.io.Serializable;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.diffplug.spotless.generic.FenceStep;
+
+class ConfigurationCacheHackListTest {
+	private static FormatterStep roundtripStep(String name, Serializable equalityState) {
+		// mirrors FormatterStep.createLazy(name, roundtripInit, equalityFunc, formatterFunc)
+		return FormatterStep.createLazy(name,
+				() -> "roundtrip-" + name,
+				rt -> equalityState,
+				eq -> (FormatterFunc) (s -> s));
+	}
+
+	private static FormatterStep toggleOffOn(FormatterStep... within) {
+		return FenceStep.named(FenceStep.defaultToggleName())
+				.openClose(FenceStep.defaultToggleOff(), FenceStep.defaultToggleOn())
+				.preserveWithin(List.of(within));
+	}
+
+	/** Gradle fingerprints the task's input (a {@link ConfigurationCacheHackList}) by calling hashCode(). */
+	private static int fingerprint(FormatterStep... steps) {
+		ConfigurationCacheHackList list = ConfigurationCacheHackList.forEquality();
+		list.addAll(List.of(steps));
+		return list.hashCode();
+	}
+
+	@Test
+	void plainStepCanBeFingerprinted() {
+		fingerprint(roundtripStep("plain", "eq-state"));
+	}
+
+	@Test
+	void fenceWrappingStepCanBeFingerprinted() {
+		fingerprint(toggleOffOn(roundtripStep("inner", "eq-state")));
+	}
+
+	/**
+	 * A sub-step whose equality state is null (e.g. a not-yet-provisioned promised state, as with
+	 * greclipse in a clean CI environment) must not break Gradle's input fingerprinting. See #2950.
+	 */
+	@Test
+	void fenceWrappingStepWithNullEqualityStateCanBeFingerprinted() {
+		fingerprint(toggleOffOn(roundtripStep("inner-null-eq", null)));
+	}
+}
diff --git a/plugin-gradle/CHANGES.md b/plugin-gradle/CHANGES.md
index d0fde86d5f..e191087703 100644
--- a/plugin-gradle/CHANGES.md
+++ b/plugin-gradle/CHANGES.md
@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `3.27.0`).
 
 ## [Unreleased]
+### Fixed
+- Fixed `spotlessGroovyGradle`/`greclipse()` (and any `toggleOffOn()`-wrapped step) failing during Gradle's input fingerprinting with "If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null" — a regression introduced in 8.6.0 that surfaces in clean CI environments. ([#2950](https://github.com/diffplug/spotless/issues/2950))
 
 ## [8.6.0] - 2026-05-27
 ### Added