karma: sockets.forEach is not a function

Running karma 0.13.16 I encountered the following issue after the last test run successfully:

Missing error handler on `socket`.
TypeError: sockets.forEach is not a function
at disconnectBrowsers (/project-path/node_modules/karma/lib/server.js:314:13)
at [object Object].<anonymous> (/project-path/node_modules/karma/lib/server.js:291:7)
at emitTwo (events.js:92:20)
at [object Object].emit (events.js:172:7)
at emitRunCompleteIfAllBrowsersDone (/project-path/node_modules/karma/lib/server.js:256:12)
at [object Object].<anonymous> (/project-path/node_modules/karma/lib/server.js:278:9)
at emitTwo (events.js:92:20)
at [object Object].emit (events.js:172:7)
at [object Object].onComplete (/project-path/node_modules/karma/lib/browser.js:142:13)
at Socket.<anonymous> (/project-path/node_modules/karma/lib/events.js:13:22)
at emitTwo (events.js:92:20)
at Socket.emit (events.js:172:7)
at Socket.onevent (/project-path/node_modules/socket.io/lib/socket.js:335:8)
at Socket.onpacket (/project-path/node_modules/socket.io/lib/socket.js:295:12)
at Client.ondecoded (/project-path/node_modules/socket.io/lib/client.js:193:14)
at Decoder.Emitter.emit (/project-path/node_modules/component-emitter/index.js:134:20)
at Decoder.add (/project-path/node_modules/socket.io-parser/index.js:247:12)
at Client.ondata (/project-path/node_modules/socket.io/lib/client.js:175:18)
at emitOne (events.js:77:13)
at Socket.emit (events.js:169:7)
at Socket.onPacket (/project-path/node_modules/engine.io/lib/socket.js:101:14)
at emitOne (events.js:77:13)
at WebSocket.emit (events.js:169:7)
at WebSocket.Transport.onPacket (/project-path/node_modules/engine.io/lib/transport.js:93:8)
at WebSocket.Transport.onData (/project-path/node_modules/engine.io/lib/transport.js:104:8)
at WebSocket.onData (/project-path/node_modules/engine.io/lib/transports/websocket.js:76:30)
at emitTwo (events.js:87:13)
at WebSocket.emit (events.js:172:7)
at Receiver.ontext (/project-path/node_modules/ws/lib/WebSocket.js:816:10)
at Receiver.parse (/project-path/node_modules/ws/lib/Receiver.hixie.js:145:8)
at doAdd (/project-path/node_modules/ws/lib/Receiver.hixie.js:104:19)
at Receiver.add (/project-path/node_modules/ws/lib/Receiver.hixie.js:108:22)
at Socket.realHandler (/project-path/node_modules/ws/lib/WebSocket.js:800:20)
at emitOne (events.js:77:13)
at Socket.emit (events.js:169:7)
at readableAddChunk (_stream_readable.js:146:16)
at Socket.Readable.push (_stream_readable.js:110:10)
at TCP.onread (net.js:523:20)

05 01 2016 23:59:46.725:WARN [PhantomJS 1.9.8 (Linux 0.0.0)]: Disconnected (1 times), because no message in 10000 ms.
JS 1.9.8 (Linux 0.0.0): Executed 42 of 42 DISCONNECTED (10.248 secs / 0.329 secs)
05 01 2016 23:59:46.726:ERROR [karma]: [TypeError: sockets.forEach is not a function]
TypeError: sockets.forEach is not a function
at disconnectBrowsers (/project-path/node_modules/karma/lib/server.js:314:13)
at [object Object].<anonymous> (/project-path/node_modules/karma/lib/server.js:291:7)
at emitTwo (events.js:92:20)
at [object Object].emit (events.js:172:7)
at emitRunCompleteIfAllBrowsersDone (/project-path/node_modules/karma/lib/server.js:256:12)
at [object Object].<anonymous> (/project-path/node_modules/karma/lib/server.js:278:9)
at emitOne (events.js:82:20)
at [object Object].emit (events.js:169:7)
at [object Object]._onTimeout (/project-path/node_modules/karma/lib/browser.js:50:15)
at Timer.listOnTimeout (timers.js:92:15)

/project-path/node_modules/karma/lib/server.js:314
sockets.forEach(function (socket) {
^

TypeError: sockets.forEach is not a function
at disconnectBrowsers (/project-path/node_modules/karma/lib/server.js:314:13)
at process.<anonymous> (/project-path/node_modules/karma/lib/server.js:355:5)
at emitOne (events.js:77:13)
at process.emit (events.js:169:7)
at process._fatalException (node.js:234:26)

I didn’t make any changes to the code or to the test itself but reinstalled the node_modules.