diff --git a/Userland/Libraries/LibC/string.cpp b/Userland/Libraries/LibC/string.cpp
index 3847afb6dc..5adfe173ae 100644
--- a/Userland/Libraries/LibC/string.cpp
+++ b/Userland/Libraries/LibC/string.cpp
@@ -357,7 +357,7 @@ int sys_nerr = EMAXERRNO;
 int strerror_r(int errnum, char* buf, size_t buflen)
 {
     auto saved_errno = errno;
-    if (errnum >= EMAXERRNO) {
+    if (errnum < 0 || errnum >= EMAXERRNO) {
         auto rc = strlcpy(buf, "unknown error", buflen);
         if (rc >= buflen)
             dbgln("strerror_r(): Invalid error number '{}' specified and the output buffer is too small.", errnum);