--- a/js/src/ctypes/Library.cpp
+++ b/js/src/ctypes/Library.cpp
@@ -157,19 +157,25 @@ Library::Create(JSContext* cx, HandleVal
   if (!library) {
 #define MAX_ERROR_LEN 1024
     char error[MAX_ERROR_LEN] = "Cannot get error from NSPR.";
     uint32_t errorLen = PR_GetErrorTextLength();
     if (errorLen && errorLen < MAX_ERROR_LEN)
       PR_GetErrorText(error);
 #undef MAX_ERROR_LEN
 
-    JSAutoByteString pathCharsUTF8;
-    if (pathCharsUTF8.encodeUtf8(cx, pathStr))
-      JS_ReportErrorUTF8(cx, "couldn't open library %s: %s", pathCharsUTF8.ptr(), error);
+    if (JS::StringIsASCII(error)) {
+      JSAutoByteString pathCharsUTF8;
+      if (pathCharsUTF8.encodeUtf8(cx, pathStr))
+        JS_ReportErrorUTF8(cx, "couldn't open library %s: %s", pathCharsUTF8.ptr(), error);
+    } else {
+      JSAutoByteString pathCharsLatin1;
+      if (pathCharsLatin1.encodeLatin1(cx, pathStr))
+        JS_ReportErrorLatin1(cx, "couldn't open library %s: %s", pathCharsLatin1.ptr(), error);
+    }
     return nullptr;
   }
 
   // stash the library
   JS_SetReservedSlot(libraryObj, SLOT_LIBRARY, PrivateValue(library));
 
   return libraryObj;
 }