--- a/netwerk/base/src/nsProtocolProxyService.cpp
+++ b/netwerk/base/src/nsProtocolProxyService.cpp
@@ -814,16 +814,18 @@ nsProtocolProxyService::ReloadPAC()
     return NS_OK;
 }
 
 // nsIProtocolProxyService
 NS_IMETHODIMP
 nsProtocolProxyService::Resolve(nsIURI *uri, uint32_t flags,
                                 nsIProxyInfo **result)
 {
+    NS_ENSURE_ARG_POINTER(uri);
+
     nsProtocolInfo info;
     nsresult rv = GetProtocolInfo(uri, &info);
     if (NS_FAILED(rv))
         return rv;
 
     bool usePAC;
     rv = Resolve_Internal(uri, info, flags, &usePAC, result);
     if (NS_FAILED(rv)) {
@@ -859,16 +861,19 @@ nsProtocolProxyService::Resolve(nsIURI *
     return NS_OK;
 }
 
 NS_IMETHODIMP
 nsProtocolProxyService::AsyncResolve(nsIURI *uri, uint32_t flags,
                                      nsIProtocolProxyCallback *callback,
                                      nsICancelable **result)
 {
+    NS_ENSURE_ARG_POINTER(uri);
+    NS_ENSURE_ARG_POINTER(callback);
+
     nsRefPtr<nsAsyncResolveRequest> ctx =
         new nsAsyncResolveRequest(this, uri, flags, callback);
     if (!ctx)
         return NS_ERROR_OUT_OF_MEMORY;
 
     nsProtocolInfo info;
     nsresult rv = GetProtocolInfo(uri, &info);
     if (NS_FAILED(rv))
@@ -1168,16 +1173,19 @@ nsProtocolProxyService::LoadHostFilters(
 loser:
         delete hinfo;
     }
 }
 
 nsresult
 nsProtocolProxyService::GetProtocolInfo(nsIURI *uri, nsProtocolInfo *info)
 {
+    NS_PRECONDITION(uri, "URI is null");
+    NS_PRECONDITION(info, "info is null");
+
     nsresult rv;
 
     rv = uri->GetScheme(info->scheme);
     if (NS_FAILED(rv))
         return rv;
 
     nsCOMPtr<nsIIOService> ios = do_GetIOService(&rv);
     if (NS_FAILED(rv))