diff --git a/common/content/commands.js b/common/content/commands.js
index ce4b8a4a..11c3a5aa 100644
--- a/common/content/commands.js
+++ b/common/content/commands.js
@@ -141,6 +141,11 @@ const Command = Class("Command", {
         let self = this;
         modifiers = modifiers || {};
 
+        if (args.count != null && !this.count)
+            throw FailedAssertion("E481: No range allowed");
+        if (args.bang && !this.bang)
+            throw FailedAssertion("E477: No ! allowed");
+
         dactyl.trapErrors(function exec(command) {
             this.action(args, modifiers);
         }, this);
diff --git a/common/content/dactyl.js b/common/content/dactyl.js
index 95722cf6..9bb72e6c 100644
--- a/common/content/dactyl.js
+++ b/common/content/dactyl.js
@@ -393,18 +393,9 @@ const Dactyl = Module("dactyl", {
 
         let err = null;
         for (let [command, args] in commands.parseCommands(str.replace(/^'(.*)'$/, "$1"))) {
-            if (command === null) {
-                err = "E492: Not a " + config.name + " command: " + str;
-                dactyl.focusContent();
-            }
-            else if (command.action === null)
-                err = "E666: Internal error: command.action === null"; // TODO: need to perform this test? -- djk
-            else if (args.count != null && !command.count)
-                err = "E481: No range allowed";
-            else if (args.bang && !command.bang)
-                err = "E477: No ! allowed";
+            if (command === null)
+                throw FailedAssertion("E492: Not a " + config.appName + " command: " + str);
 
-            dactyl.assert(!err, err);
             if (!silent)
                 commandline.command = str.replace(/^\s*:\s*/, "");
 
diff --git a/common/content/io.js b/common/content/io.js
index e1b1cd72..3b2ba161 100644
--- a/common/content/io.js
+++ b/common/content/io.js
@@ -386,7 +386,14 @@ lookup:
                     line = line.replace(/\r$/, "");
 
                     if (!/^\s*(".*)?$/.test(line))
-                        dactyl.execute(line, { setFrom: file }, true);
+                        try {
+                            dactyl.execute(line, { setFrom: file }, true);
+                        }
+                        catch (e) {
+                            dactyl.echoerr("Error detected while processing " + file.path);
+                            dactyl.echomsg("line\t" + this.sourcing.line + ":");
+                            dactyl.reportError(e, true);
+                        }
                 }
             }