diff --git a/core/libs/prx/libc/src/Errors.cpp b/core/libs/prx/libc/src/Errors.cpp
index 963191cf6..5523e1bf7 100644
--- a/core/libs/prx/libc/src/Errors.cpp
+++ b/core/libs/prx/libc/src/Errors.cpp
@@ -54,7 +54,7 @@ int APS5_VABI strerror_r_nid_postfix(int error, char* buffer, std::size_t length
     else std::snprintf(temporary, sizeof(temporary), "Unknown error: %d", error);
     const auto required = std::strlen(temporary) + 1;
     int result = Known(error) ? 0 : 22;
-    if (!buffer || length < required) result = 34;
+    if (Known(error) && (!buffer || length < required)) result = 34;
     if (buffer && length) {
         const auto count = required <= length ? required - 1 : length - 1;
         std::memcpy(buffer, temporary, count);
diff --git a/core/libs/tests/GuestErrors.cpp b/core/libs/tests/GuestErrors.cpp
index 52160d562..fd2c8a422 100644
--- a/core/libs/tests/GuestErrors.cpp
+++ b/core/libs/tests/GuestErrors.cpp
@@ -36,4 +36,7 @@ int main() {
     sentinel[0] = 'x';
     Require(strerror_r_nid_postfix(22, sentinel, 0) == 34 && sentinel[0] == 'x');
     Require(strerror_r_nid_postfix(22, nullptr, 0) == 34);
+    char truncated[4];
+    Require(strerror_r_nid_postfix(97, truncated, sizeof(truncated)) == 22 && std::strcmp(truncated, "Unk") == 0);
+    Require(strerror_r_nid_postfix(-1, truncated, sizeof(truncated)) == 22 && std::strcmp(truncated, "Unk") == 0);
 }