diff --git a/Source/JavaScriptCore/runtime/ErrorInstance.cpp b/Source/JavaScriptCore/runtime/ErrorInstance.cpp
index 0f090b58c1e95..ae122859be500 100644
--- a/Source/JavaScriptCore/runtime/ErrorInstance.cpp
+++ b/Source/JavaScriptCore/runtime/ErrorInstance.cpp
@@ -396,7 +396,7 @@ void ErrorInstance::computeErrorInfo(VM& vm, bool allocationAllowed)
             if (m_stackPropertyAlreadyMaterialized)
                 stackString = emptyString();
             else
-                stackString = fn(vm, *m_stackTrace.get(), m_lineColumn.line, m_lineColumn.column, m_sourceURL, this->bunErrorData());
+                stackString = fn(vm, *m_stackTrace.get(), m_lineColumn.line, m_lineColumn.column, m_sourceURL, this, this->bunErrorData());
         } else {
             getLineColumnAndSource(vm, m_stackTrace.get(), m_lineColumn, m_sourceURL);
             // If the stack property was already materialized by Error.captureStackString,
diff --git a/Source/JavaScriptCore/runtime/VM.h b/Source/JavaScriptCore/runtime/VM.h
index cf23d183cee9d..0709cec3af9c5 100644
--- a/Source/JavaScriptCore/runtime/VM.h
+++ b/Source/JavaScriptCore/runtime/VM.h
@@ -173,7 +173,11 @@ constexpr bool validateDFGDoesGC = ENABLE_DFG_DOES_GC_VALIDATION;
 
 #if USE(BUN_JSC_ADDITIONS)
 using StackTraceAppenderFunction = WTF::Function<void(VM&, JSCell* owner, Vector<StackFrame>& stackTrace, size_t maxToAppend)>;
-using ErrorInfoFunction = WTF::Function<String(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, void* bunErrorData)>;
+// Called by ErrorInstance::computeErrorInfo. Its main caller is ErrorInstance::reconcileWeakReferencesAtGCEnd,
+// which runs inside Heap::runEndPhase when a frame of a not yet materialized stack trace died; the returned
+// string then becomes the error's final .stack. errorInstance is the error being formatted, so the embedder
+// can still put its name and message on the first line. The function must not allocate GC cells or run JS.
+using ErrorInfoFunction = WTF::Function<String(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, JSC::JSObject* errorInstance, void* bunErrorData)>;
 using ErrorInfoFunctionJSValue = WTF::Function<JSValue(VM&, Vector<StackFrame>& stackTrace, unsigned& line, unsigned& column, String& sourceURL, JSC::JSObject*, void* bunErrorData)>;
 #endif