diff --git a/modules/libpr0n/src/imgRequest.cpp b/modules/libpr0n/src/imgRequest.cpp
index 5f387b27d78a..60b04fbab4db 100644
--- a/modules/libpr0n/src/imgRequest.cpp
+++ b/modules/libpr0n/src/imgRequest.cpp
@@ -676,7 +676,7 @@ NS_IMETHODIMP imgRequest::OnDataAvailable(nsIRequest *aRequest, nsISupports *ctx
     nsCAutoString conid("@mozilla.org/image/decoder;2?type=");
     conid += mContentType;
 
-    mDecoder = do_CreateInstance(conid);
+    mDecoder = do_CreateInstance(conid.get());
 
     if (!mDecoder) {
       PR_LOG(gImgLog, PR_LOG_WARNING,