--- a/toolkit/components/viewsource/content/viewSource-content.js
+++ b/toolkit/components/viewsource/content/viewSource-content.js
@@ -286,17 +286,19 @@ var ViewSourceContent = {
    *        loading.
    * @param forcedCharSet (optional)
    *        The document character set to use instead of the default one.
    */
   loadSource(URL, pageDescriptor, lineNumber, forcedCharSet) {
     const viewSrcURL = "view-source:" + URL;
 
     if (forcedCharSet) {
-      docShell.charset = forcedCharSet;
+      try {
+        docShell.charset = forcedCharSet;
+      } catch (e) { /* invalid charset */ }
     }
 
     if (lineNumber && lineNumber > 0) {
       let doneLoading = (event) => {
         // Ignore possible initial load of about:blank
         if (this.isAboutBlank ||
             !content.document.body) {
           return;