diff --git a/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts b/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
index 11014299ff5..6df4312e6f6 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadDebugService.ts
@@ -14,9 +14,8 @@ import {
 import { extHostNamedCustomer } from 'vs/workbench/api/electron-browser/extHostCustomers';
 import severity from 'vs/base/common/severity';
 import { AbstractDebugAdapter } from 'vs/workbench/parts/debug/node/debugAdapter';
-import * as paths from 'vs/base/common/paths';
 import { IWorkspaceFolder } from 'vs/platform/workspace/common/workspace';
-import { convertToVSCPaths, convertToDAPaths } from 'vs/workbench/parts/debug/common/debugUtils';
+import { convertToVSCPaths, convertToDAPaths, stringToUri, uriToString } from 'vs/workbench/parts/debug/common/debugUtils';
 import { deepClone } from 'vs/base/common/objects';
 
 
@@ -215,11 +214,7 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 
 	public $acceptDAMessage(handle: number, message: DebugProtocol.ProtocolMessage) {
 
-		convertToVSCPaths(message, source => {
-			if (typeof source.path === 'object') {
-				source.path = uri.revive(source.path).toString();
-			}
-		});
+		convertToVSCPaths(message, source => uriToString(source));
 
 		this._debugAdapters.get(handle).acceptMessage(message);
 	}
@@ -302,13 +297,7 @@ class ExtensionHostDebugAdapter extends AbstractDebugAdapter {
 		// since we modify Source.paths in the message in place, we need to make a copy of it (see #61129)
 		const msg = deepClone(message);
 
-		convertToDAPaths(msg, source => {
-			if (paths.isAbsolute(source.path)) {
-				(<any>source).path = uri.file(source.path);
-			} else {
-				(<any>source).path = uri.parse(source.path);
-			}
-		});
+		convertToDAPaths(msg, source => stringToUri(source));
 
 		this._proxy.$sendDAMessage(this._handle, msg);
 	}
diff --git a/src/vs/workbench/api/node/extHostDebugService.ts b/src/vs/workbench/api/node/extHostDebugService.ts
index 46ee6e9e4aa..6552de5fc9e 100644
--- a/src/vs/workbench/api/node/extHostDebugService.ts
+++ b/src/vs/workbench/api/node/extHostDebugService.ts
@@ -27,7 +27,7 @@ import { getTerminalLauncher, hasChildprocesses, prepareCommand } from 'vs/workb
 import { IWorkspaceFolder } from 'vs/platform/workspace/common/workspace';
 import { AbstractVariableResolverService } from 'vs/workbench/services/configurationResolver/node/variableResolver';
 import { ExtHostConfiguration } from './extHostConfiguration';
-import { convertToVSCPaths, convertToDAPaths } from 'vs/workbench/parts/debug/common/debugUtils';
+import { convertToVSCPaths, convertToDAPaths, stringToUri, uriToString } from 'vs/workbench/parts/debug/common/debugUtils';
 import { ExtHostTerminalService } from 'vs/workbench/api/node/extHostTerminalService';
 import { IDisposable } from 'vs/base/common/lifecycle';
 import { IConfigurationResolverService } from 'vs/workbench/services/configurationResolver/common/configurationResolver';
@@ -400,11 +400,7 @@ export class ExtHostDebugService implements ExtHostDebugServiceShape {
 						}
 
 						// DA -> VS Code
-						convertToVSCPaths(msg, source => {
-							if (paths.isAbsolute(source.path)) {
-								(<any>source).path = URI.file(source.path);
-							}
-						});
+						convertToVSCPaths(msg, source => stringToUri(source));
 
 						mythis._debugServiceProxy.$acceptDAMessage(handle, msg);
 					});
@@ -435,11 +431,7 @@ export class ExtHostDebugService implements ExtHostDebugServiceShape {
 
 	public $sendDAMessage(handle: number, message: DebugProtocol.ProtocolMessage): TPromise<void> {
 		// VS Code -> DA
-		convertToDAPaths(message, source => {
-			if (typeof source.path === 'object') {
-				source.path = URI.revive(source.path).fsPath;
-			}
-		});
+		convertToDAPaths(message, source => uriToString(source));
 
 		const tracker = this._debugAdaptersTrackers.get(handle);
 		if (tracker) {
diff --git a/src/vs/workbench/parts/debug/common/debugSource.ts b/src/vs/workbench/parts/debug/common/debugSource.ts
index a3a322880c6..992a1526095 100644
--- a/src/vs/workbench/parts/debug/common/debugSource.ts
+++ b/src/vs/workbench/parts/debug/common/debugSource.ts
@@ -12,6 +12,7 @@ import { DEBUG_SCHEME } from 'vs/workbench/parts/debug/common/debug';
 import { IRange } from 'vs/editor/common/core/range';
 import { IEditorService, SIDE_GROUP, ACTIVE_GROUP } from 'vs/workbench/services/editor/common/editorService';
 import { Schemas } from 'vs/base/common/network';
+import { isUri } from 'vs/workbench/parts/debug/common/debugUtils';
 
 const UNKNOWN_SOURCE_LABEL = nls.localize('unknownSource', "Unknown Source");
 
@@ -36,23 +37,29 @@ export class Source {
 
 	constructor(public raw: DebugProtocol.Source, sessionId: string) {
 		let path: string;
-		if (!raw) {
+		if (raw) {
+			path = this.raw.path || this.raw.name;
+			this.available = true;
+		} else {
 			this.raw = { name: UNKNOWN_SOURCE_LABEL };
 			this.available = false;
 			path = `${DEBUG_SCHEME}:${UNKNOWN_SOURCE_LABEL}`;
-		} else {
-			path = this.raw.path || this.raw.name;
-			this.available = true;
 		}
 
 		if (this.raw.sourceReference > 0) {
 			this.uri = uri.parse(`${DEBUG_SCHEME}:${encodeURIComponent(path)}?session=${encodeURIComponent(sessionId)}&ref=${this.raw.sourceReference}`);
 		} else {
-			if (paths.isAbsolute(path)) {
-				this.uri = uri.file(path);
-			} else {
-				// assume that path is a URI
+			if (isUri(path)) {
 				this.uri = uri.parse(path);
+			} else {
+				// assume path
+				if (paths.isAbsolute_posix(path) || paths.isAbsolute_win32(path)) {
+					this.uri = uri.file(path);
+				} else {
+					// path is relative
+					// should not happen because relative paths always have a sourceReference > 0
+					console.error('cannot handle relative paths without sourceReference');
+				}
 			}
 		}
 	}
diff --git a/src/vs/workbench/parts/debug/common/debugUtils.ts b/src/vs/workbench/parts/debug/common/debugUtils.ts
index f2be8a2b233..24091e11889 100644
--- a/src/vs/workbench/parts/debug/common/debugUtils.ts
+++ b/src/vs/workbench/parts/debug/common/debugUtils.ts
@@ -5,6 +5,8 @@
 
 import { equalsIgnoreCase } from 'vs/base/common/strings';
 import { IConfig } from 'vs/workbench/parts/debug/common/debug';
+import { URI as uri } from 'vs/base/common/uri';
+import { isAbsolute_posix, isAbsolute_win32 } from 'vs/base/common/paths';
 
 const _formatPIIRegexp = /{([^}]+)}/g;
 
@@ -67,6 +69,41 @@ export function getExactExpressionStartAndEnd(lineContent: string, looseStart: n
 		{ start: 0, end: 0 };
 }
 
+// RFC 2396, Appendix A: https://www.ietf.org/rfc/rfc2396.txt
+const _schemePattern = /^[a-zA-Z][a-zA-Z0-9\+\-\.]+:/;
+
+export function isUri(s: string) {
+	// heuristics: a valid uri starts with a scheme and
+	// the scheme has at least 2 characters so that it doesn't look like a drive letter.
+	return s && s.match(_schemePattern);
+}
+
+export function stringToUri(source: DebugProtocol.Source): void {
+	if (typeof source.path === 'string') {
+		if (isUri(source.path)) {
+			(<any>source).path = uri.parse(source.path);
+		} else {
+			// assume path
+			if (isAbsolute_posix(source.path) || isAbsolute_win32(source.path)) {
+				(<any>source).path = uri.file(source.path);
+			} else {
+				// leave relative path as is
+			}
+		}
+	}
+}
+
+export function uriToString(source: DebugProtocol.Source): void {
+	if (typeof source.path === 'object') {
+		const u = uri.revive(source.path);
+		if (u.scheme === 'file') {
+			source.path = u.fsPath;
+		} else {
+			source.path = u.toString();
+		}
+	}
+}
+
 // path hooks helpers
 
 export function convertToDAPaths(msg: DebugProtocol.ProtocolMessage, fixSourcePaths: (source: DebugProtocol.Source) => void): void {