diff --git a/core/libs/prx/libSceFiber/Export.cpp b/core/libs/prx/libSceFiber/Export.cpp
index de28742c01..b70e32786d 100644
--- a/core/libs/prx/libSceFiber/Export.cpp
+++ b/core/libs/prx/libSceFiber/Export.cpp
@@ -624,7 +624,13 @@ int32_t APS5_VABI sceFiberRename(FiberObject* object, const char* name) {
 
 int32_t APS5_VABI sceFiberOptParamInitialize(FiberOptParam* opt_param) {
     if (!opt_param) return SCE_FIBER_ERROR_NULL;
-    std::memset(opt_param, 0, FIBER_OPT_PARAM_SIZE);
+    const std::uint8_t zero[FIBER_OPT_PARAM_SIZE] = {};
+    const auto preserved = errno;
+    if (!GuestCopyOut_nid_no_patch(opt_param, zero, sizeof(zero))) {
+        errno = preserved;
+        return SCE_FIBER_ERROR_INVALID;
+    }
+    errno = preserved;
     return SCE_OK;
 }
 
diff --git a/core/libs/tests/Fiber.cpp b/core/libs/tests/Fiber.cpp
index d4eedb1c3a..7850d50164 100644
--- a/core/libs/tests/Fiber.cpp
+++ b/core/libs/tests/Fiber.cpp
@@ -566,6 +566,23 @@ int main(int argc, char** argv) {
 #endif
     FiberObject* self = nullptr;
     Require(sceFiberGetSelf(&self) == FiberErrorPermission, "no fiber on the thread");
+    Require(sceFiberOptParamInitialize(nullptr) == FiberErrorNull, "null opt param");
+    Require(sceFiberOptParamInitialize(reinterpret_cast<FiberOptParam*>(0x10)) == FiberErrorInvalid, "unwritable opt param");
+    struct alignas(8) AlignedOptParamBuffer {
+        std::uint8_t bytes[0x80];
+        std::uint32_t sentinel = 0x5a5a5a5a;
+    } storage;
+    std::memset(storage.bytes, 0xA5, sizeof(storage.bytes));
+    Require(sceFiberOptParamInitialize(reinterpret_cast<FiberOptParam*>(storage.bytes)) == 0, "initialize opt param");
+    bool allZero = true;
+    for (std::size_t i = 0; i < sizeof(storage.bytes); ++i) {
+        if (storage.bytes[i] != 0) {
+            allZero = false;
+            break;
+        }
+    }
+    Require(allZero, "opt param cleared");
+    Require(storage.sentinel == 0x5a5a5a5a, "sentinel intact");
     Require(_sceFiberInitializeImpl_nid_postfix(g_first, "first", FirstEntry, 11, g_firstContext.data(), g_firstContext.size(), nullptr, 0) == 0, "initialize first");
     Require(_sceFiberInitializeImpl_nid_postfix(g_second, "second", SecondEntry, 22, g_secondContext.data(), g_secondContext.size(), nullptr, 0) == 0, "initialize second");
     const auto threadCsr = _mm_getcsr();
diff --git a/docs/dev/debt/libSceFiber.md b/docs/dev/debt/libSceFiber.md
index da142ae02b..14aca4f493 100644
--- a/docs/dev/debt/libSceFiber.md
+++ b/docs/dev/debt/libSceFiber.md
@@ -2,6 +2,7 @@
 
 ### Functional
 
+- [sceFiberOptParamInitialize](../../../core/libs/prx/libSceFiber/Export.cpp) returns SCE_FIBER_ERROR_INVALID when the destination pointer is unwritable. This is a defensive host-protection fallback; whether the console raises a signal or faults on unmapped or read-only memory is not measured.
 - [sceFiberGetInfo](../../../core/libs/prx/libSceFiber/Export.cpp) reports the requested-size window at the top of the host stack used for execution, rather than the original caller buffer, which is retained and pinned. The additional 1 MiB host-call margin below that window is not included in `size_context`. Planet Coaster (PPSA01736 v01.017) uses this range for ChakraCore garbage-collector stack scans; reporting the unused caller buffer makes the scan cross unmapped memory. The range is covered by a regression test while running, switching and migrating a fiber between threads. Console behavior for substituted host stacks has no direct equivalent and is not measured.
 
 ### Unknown function info