diff --git a/dom/media/MediaDevices.cpp b/dom/media/MediaDevices.cpp
index fd5649dd1ed2..cde504b897ad 100644
--- a/dom/media/MediaDevices.cpp
+++ b/dom/media/MediaDevices.cpp
@@ -52,31 +52,34 @@ public:
   OnSuccess(nsIVariant* aDevices) override
   {
     // Cribbed from MediaPermissionGonk.cpp
-    nsIID elementIID;
-    uint16_t elementType;
 
     // Create array for nsIMediaDevice
     nsTArray<nsCOMPtr<nsIMediaDevice>> devices;
     // Contain the fumes
     {
-      void* rawArray;
-      uint32_t arrayLen;
-      nsresult rv;
-      rv = aDevices->GetAsArray(&elementType, &elementIID, &arrayLen, &rawArray);
+      uint16_t vtype;
+      nsresult rv = aDevices->GetDataType(&vtype);
       NS_ENSURE_SUCCESS(rv, rv);
+      if (vtype != nsIDataType::VTYPE_EMPTY_ARRAY) {
+        nsIID elementIID;
+        uint16_t elementType;
+        void* rawArray;
+        uint32_t arrayLen;
+        rv = aDevices->GetAsArray(&elementType, &elementIID, &arrayLen, &rawArray);
+        NS_ENSURE_SUCCESS(rv, rv);
+        if (elementType != nsIDataType::VTYPE_INTERFACE) {
+          free(rawArray);
+          return NS_ERROR_FAILURE;
+        }
 
-      if (elementType != nsIDataType::VTYPE_INTERFACE) {
-        free(rawArray);
-        return NS_ERROR_FAILURE;
+        nsISupports **supportsArray = reinterpret_cast<nsISupports **>(rawArray);
+        for (uint32_t i = 0; i < arrayLen; ++i) {
+          nsCOMPtr<nsIMediaDevice> device(do_QueryInterface(supportsArray[i]));
+          devices.AppendElement(device);
+          NS_IF_RELEASE(supportsArray[i]); // explicitly decrease refcount for rawptr
+        }
+        free(rawArray); // explicitly free memory from nsIVariant::GetAsArray
       }
-
-      nsISupports **supportsArray = reinterpret_cast<nsISupports **>(rawArray);
-      for (uint32_t i = 0; i < arrayLen; ++i) {
-        nsCOMPtr<nsIMediaDevice> device(do_QueryInterface(supportsArray[i]));
-        devices.AppendElement(device);
-        NS_IF_RELEASE(supportsArray[i]); // explicitly decrease refcount for rawptr
-      }
-      free(rawArray); // explicitly free memory from nsIVariant::GetAsArray
     }
     nsTArray<nsRefPtr<MediaDeviceInfo>> infos;
     for (auto& device : devices) {
diff --git a/dom/media/tests/mochitest/test_enumerateDevices.html b/dom/media/tests/mochitest/test_enumerateDevices.html
index 9582d91c15fa..6b623d1b7989 100644
--- a/dom/media/tests/mochitest/test_enumerateDevices.html
+++ b/dom/media/tests/mochitest/test_enumerateDevices.html
@@ -21,10 +21,10 @@ function mustFailWith(msg, reason, f) {
                   e => is(e.name, reason, msg + " must fail: " + e.message));
 }
 
-var pushPrefs = dict => new Promise(res => SpecialPowers.pushPrefEnv(dict, res));
+var pushPrefs = (...p) => new Promise(r => SpecialPowers.pushPrefEnv({set: p}, r));
 
 runTest(() =>
-  pushPrefs({ set : [["media.navigator.streams.fake", true]] })
+  pushPrefs(["media.navigator.streams.fake", true])
   .then(() => navigator.mediaDevices.enumerateDevices())
   .then(devices => {
     ok(devices.length > 0, "At least one device found");
@@ -58,7 +58,12 @@ runTest(() =>
                        () => navigator.mediaDevices.getUserMedia({
     audio: { deviceId: { exact: "unknown9qHr8B0JIbcHlbl9xR+jMbZZ8WyoPfpCXPfc=" } },
     fake: true,
-  }))));
+  })))
+  // Check the special case of no devices found (these prefs override fake).
+  .then(() => pushPrefs(["media.audio_loopback_dev", "none"],
+                        ["media.video_loopback_dev", "none"]))
+  .then(() => navigator.mediaDevices.enumerateDevices())
+  .then(devices => ok(devices.length === 0, "No devices found")));
 
 </script>
 </pre>