diff --git a/mozilla/extensions/schema-validation/src/nsSchemaLoader.cpp b/mozilla/extensions/schema-validation/src/nsSchemaLoader.cpp
index 51c985be89f..0e086a687a7 100644
--- a/mozilla/extensions/schema-validation/src/nsSchemaLoader.cpp
+++ b/mozilla/extensions/schema-validation/src/nsSchemaLoader.cpp
@@ -43,7 +43,7 @@
 // XPConnect includes
 #include "nsIXPConnect.h"
 #include "nsIScriptSecurityManager.h"
-#include "nsIPrincipal.h"
+#include "nsICodebasePrincipal.h"
 
 // XPCOM includes
 #include "nsIServiceManager.h"
@@ -813,7 +813,10 @@ nsSchemaLoader::GetResolvedURI(const nsAString& aSchemaURI,
     nsCOMPtr<nsIPrincipal> principal;
     rv = secMan->GetSubjectPrincipal(getter_AddRefs(principal));
     if (NS_SUCCEEDED(rv)) {
-      principal->GetURI(getter_AddRefs(baseURI));
+      nsCOMPtr<nsICodebasePrincipal> codebase = do_QueryInterface(principal);
+      if (codebase) {
+        codebase->GetURI(getter_AddRefs(baseURI));
+      }
     }
     
     rv = NS_NewURI(aURI, aSchemaURI, nsnull, baseURI);