diff --git a/src/vs/workbench/services/extensions/common/abstractExtensionService.ts b/src/vs/workbench/services/extensions/common/abstractExtensionService.ts
index bcfa887b1a462..8bd6d25f2ca96 100644
--- a/src/vs/workbench/services/extensions/common/abstractExtensionService.ts
+++ b/src/vs/workbench/services/extensions/common/abstractExtensionService.ts
@@ -443,7 +443,7 @@ export abstract class AbstractExtensionService extends Disposable implements IEx
 
 class ProposedApiController {
 
-	private readonly enableProposedApiFor: string | string[];
+	private readonly enableProposedApiFor: string[];
 	private readonly enableProposedApiForAll: boolean;
 	private readonly productAllowProposedApi: Set<string>;
 
@@ -451,15 +451,8 @@ class ProposedApiController {
 		@IWorkbenchEnvironmentService environmentService: IWorkbenchEnvironmentService,
 		@IProductService productService: IProductService
 	) {
-		this.enableProposedApiFor = environmentService.args['enable-proposed-api'] || [];
-		if (this.enableProposedApiFor.length) {
-			// Make enabled proposed API be lowercase for case insensitive comparison
-			if (Array.isArray(this.enableProposedApiFor)) {
-				this.enableProposedApiFor = this.enableProposedApiFor.map(id => id.toLowerCase());
-			} else {
-				this.enableProposedApiFor = this.enableProposedApiFor.toLowerCase();
-			}
-		}
+		// Make enabled proposed API be lowercase for case insensitive comparison
+		this.enableProposedApiFor = (environmentService.args['enable-proposed-api'] || []).map(id => id.toLowerCase());
 
 		this.enableProposedApiForAll = !environmentService.isBuilt ||
 			(!!environmentService.extensionDevelopmentLocationURI && productService.nameLong !== 'Visual Studio Code') ||