Throw an error when adding an invalid selector

spec/command-registry-spec.coffee

@@ -148,6 +148,16 @@ describe "CommandRegistry", ->
       grandchild.dispatchEvent(new CustomEvent('command-2', bubbles: true))
       expect(calls).toEqual []
 
+  describe "::add(selector, commandName, callback)", ->
+    it "throws an error when called with an invalid selector", ->
+      badSelector = '<>'
+      addError = null
+      try
+        registry.add badSelector, 'foo:bar', ->
+      catch error
+        addError = error
+      expect(addError.message).toContain(badSelector)
+
   describe "::findCommands({target})", ->
     it "returns commands that can be invoked on the target or its ancestors", ->
       registry.add '.parent', 'namespace:command-1', ->

src/command-registry.coffee

@@ -87,6 +87,8 @@ class CommandRegistry
       return disposable
 
     if typeof target is 'string'
+      unless @isSelectorValid(target)
+        throw new Error("'#{target}' is not a valid selector")
       @addSelectorBasedListener(target, commandName, callback)
     else
       @addInlineListener(target, commandName, callback)
@@ -235,6 +237,20 @@ class CommandRegistry
       window.addEventListener(commandName, @handleCommandEvent, true)
       @registeredCommands[commandName] = true
 
+  isSelectorValid: (selector) ->
+    @selectorCache ?= {}
+    cachedValue = @selectorCache[selector]
+    return cachedValue if cachedValue?
+
+    @testElement ?= document.createElement('div')
+    try
+      @testElement.webkitMatchesSelector(selector)
+      @selectorCache[selector] = true
+      true
+    catch selectorError
+      @selectorCache[selector] = false
+      false
+
 class SelectorBasedListener
   constructor: (@selector, @callback) ->
     @specificity = (SpecificityCache[@selector] ?= specificity(@selector))