--- a/dom/canvas/CanvasUtils.cpp
+++ b/dom/canvas/CanvasUtils.cpp
@@ -68,18 +68,17 @@ bool IsImageExtractionAllowed(nsIDocumen
 
     // Get the document URI and its spec.
     nsIURI *docURI = aDocument->GetDocumentURI();
     nsCString docURISpec;
     docURI->GetSpec(docURISpec);
 
     // Allow local files to extract canvas data.
     bool isFileURL;
-    (void) docURI->SchemeIs("file", &isFileURL);
-    if (isFileURL) {
+    if (NS_SUCCEEDED(docURI->SchemeIs("file", &isFileURL)) && isFileURL) {
         return true;
     }
 
     // Get calling script file and line for logging.
     JS::AutoFilename scriptFile;
     unsigned scriptLine = 0;
     bool isScriptKnown = false;
     if (JS::DescribeScriptedCaller(aCx, &scriptFile, &scriptLine)) {