diff --git a/src/js/plyr.js b/src/js/plyr.js
index 663df980..f1b3c83e 100644
--- a/src/js/plyr.js
+++ b/src/js/plyr.js
@@ -373,10 +373,10 @@
                 return input !== null && input instanceof Event;
             },
             cue: function(input) {
-                return input !== null && (input instanceof window.TextTrackCue || input instanceof window.VTTCue);
+                this.instanceOf(input, window.TextTrackCue) || this.instanceOf(input, window.VTTCue);
             },
             track: function(input) {
-                return input !== null && (input instanceof window.TextTrack || typeof input.kind === 'string');
+                return input !== null && (this.instanceOf(input, window.TextTrack) || typeof input.kind === 'string');
             },
             undefined: function(input) {
                 return input !== null && typeof input === 'undefined';
@@ -389,6 +389,9 @@
                     (this.object(input) && Object.keys(input).length === 0)
                 );
             },
+            instanceOf: function(input, constructor) {
+                return Boolean(input && constructor && input instanceof constructor);
+            },
         },
 
         // Credits: http://paypal.github.io/accessible-html5-video-player/
@@ -4100,12 +4103,12 @@
 
                         case 38:
                             // Arrow up
-                            player.increaseVolume();
+                            player.increaseVolume(0.1);
                             break;
 
                         case 40:
                             // Arrow down
-                            player.decreaseVolume();
+                            player.decreaseVolume(0.1);
                             break;
 
                         case 77: