diff --git a/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts b/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
index f54eed8474eb7fc7e8fe185b976dabe1058272bf..e11ea49ac335edc1eaa3fe453341ff856f79724c 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
@@ -6,7 +6,7 @@
 
 import { IDisposable, dispose } from 'vs/base/common/lifecycle';
 import uri from 'vs/base/common/uri';
-import { IDebugService, IConfig, IDebugConfigurationProvider, IBreakpoint, IFunctionBreakpoint, IBreakpointData, IAdapterExecutable, ITerminalSettings, IDebugAdapter, IDebugAdapterProvider } from 'vs/workbench/parts/debug/common/debug';
+import { IDebugService, IConfig, IDebugConfigurationProvider, IBreakpoint, IFunctionBreakpoint, IBreakpointData, IAdapterExecutable, ITerminalSettings, IDebugAdapter, IDebugAdapterProvider, ISession } from 'vs/workbench/parts/debug/common/debug';
 import { TPromise } from 'vs/base/common/winjs.base';
 import {
 	ExtHostContext, ExtHostDebugServiceShape, MainThreadDebugServiceShape, DebugSessionUUID, MainContext,
@@ -28,6 +28,7 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 	private _breakpointEventsActive: boolean;
 	private _debugAdapters: Map<number, ExtensionHostDebugAdapter>;
 	private _debugAdaptersHandleCounter = 1;
+	private _sessions: Map<DebugSessionUUID, ISession>;
 
 	constructor(
 		extHostContext: IExtHostContext,
@@ -38,7 +39,9 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 		this._toDispose.push(debugService.onDidNewSession(session => {
 			this._proxy.$acceptDebugSessionStarted(<DebugSessionUUID>session.getId(), session.configuration.type, session.getName(false));
 		}));
+		this._sessions = new Map();
 		this._toDispose.push(debugService.onWillNewSession(session => {
+			this._sessions.set(session.getId(), session);
 			// Need to start listening early to new session events because a custom event can come while a session is initialising
 			this._toDispose.push(session.onDidCustomEvent(event => {
 				if (event && event.sessionId) {
@@ -46,7 +49,10 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 				}
 			}));
 		}));
-		this._toDispose.push(debugService.onDidEndSession(proc => this._proxy.$acceptDebugSessionTerminated(<DebugSessionUUID>proc.getId(), proc.configuration.type, proc.getName(false))));
+		this._toDispose.push(debugService.onDidEndSession(session => {
+			this._proxy.$acceptDebugSessionTerminated(<DebugSessionUUID>session.getId(), session.configuration.type, session.getName(false));
+			this._sessions.delete(<DebugSessionUUID>session.getId());
+		}));
 		this._toDispose.push(debugService.getViewModel().onDidFocusSession(proc => {
 			if (proc) {
 				this._proxy.$acceptDebugSessionActiveChanged(<DebugSessionUUID>proc.getId(), proc.configuration.type, proc.getName(false));
@@ -220,9 +226,9 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 	}
 
 	public $customDebugAdapterRequest(sessionId: DebugSessionUUID, request: string, args: any): TPromise<any> {
-		const process = this.debugService.getModel().getSessions().filter(p => p.getId() === sessionId).pop();
-		if (process) {
-			return process.raw.custom(request, args).then(response => {
+		const session = this._sessions.get(sessionId);
+		if (session) {
+			return session.raw.custom(request, args).then(response => {
 				if (response && response.success) {
 					return response.body;
 				} else {