diff --git a/src/vs/workbench/common/resources.ts b/src/vs/workbench/common/resources.ts
index ea523a95f66..6acefead117 100644
--- a/src/vs/workbench/common/resources.ts
+++ b/src/vs/workbench/common/resources.ts
@@ -148,7 +148,13 @@ export class ResourceGlobMatcher {
 
 	public matches(resource: URI): boolean {
 		const root = this.contextService.getRoot(resource);
-		const expressionForRoot = this.mapRootToParsedExpression.get(root ? root.toString() : ResourceGlobMatcher.NO_ROOT);
+
+		let expressionForRoot: ParsedExpression;
+		if (root && this.mapRootToParsedExpression.has(root.toString())) {
+			expressionForRoot = this.mapRootToParsedExpression.get(root.toString());
+		} else {
+			expressionForRoot = this.mapRootToParsedExpression.get(ResourceGlobMatcher.NO_ROOT);
+		}
 
 		// If the resource if from a workspace, convert its absolute path to a relative
 		// path so that glob patterns have a higher probability to match. For example