diff --git a/src/vs/workbench/api/common/extHost.protocol.ts b/src/vs/workbench/api/common/extHost.protocol.ts
index c14f1d0b26f355cf98712244913cdbcadac3b10d..2d08ee74397518e0a1fdf76ac57a004cb77f9340 100644
--- a/src/vs/workbench/api/common/extHost.protocol.ts
+++ b/src/vs/workbench/api/common/extHost.protocol.ts
@@ -1007,7 +1007,7 @@ export interface ISuggestDataDto {
 	[ISuggestDataDtoField.documentation]?: string | IMarkdownString;
 	[ISuggestDataDtoField.sortText]?: string;
 	[ISuggestDataDtoField.filterText]?: string;
-	[ISuggestDataDtoField.preselect]?: boolean;
+	[ISuggestDataDtoField.preselect]?: true;
 	[ISuggestDataDtoField.insertText]?: string;
 	[ISuggestDataDtoField.insertTextRules]?: modes.CompletionItemInsertTextRule;
 	[ISuggestDataDtoField.range]?: IRange | { insert: IRange, replace: IRange; };
@@ -1023,8 +1023,8 @@ export interface ISuggestResultDto {
 	x?: number;
 	a: { insert: IRange, replace: IRange; };
 	b: ISuggestDataDto[];
-	c?: boolean;
-	d?: boolean;
+	c?: true;
+	d?: true;
 }
 
 export interface ISignatureHelpDto {
diff --git a/src/vs/workbench/api/common/extHostLanguageFeatures.ts b/src/vs/workbench/api/common/extHostLanguageFeatures.ts
index c440c5a3f14deb403b78c5bef6c3a55d5566f5a5..20cc1a12f77d244fc0b3abbdb3f2ea8dcfc27fc3 100644
--- a/src/vs/workbench/api/common/extHostLanguageFeatures.ts
+++ b/src/vs/workbench/api/common/extHostLanguageFeatures.ts
@@ -890,7 +890,7 @@ class SuggestAdapter {
 			[extHostProtocol.ISuggestDataDtoField.documentation]: typeof item.documentation === 'undefined' ? undefined : typeConvert.MarkdownString.fromStrict(item.documentation),
 			[extHostProtocol.ISuggestDataDtoField.sortText]: item.sortText,
 			[extHostProtocol.ISuggestDataDtoField.filterText]: item.filterText,
-			[extHostProtocol.ISuggestDataDtoField.preselect]: item.preselect,
+			[extHostProtocol.ISuggestDataDtoField.preselect]: item.preselect || undefined,
 			[extHostProtocol.ISuggestDataDtoField.insertTextRules]: item.keepWhitespace ? modes.CompletionItemInsertTextRule.KeepWhitespace : 0,
 			[extHostProtocol.ISuggestDataDtoField.commitCharacters]: item.commitCharacters,
 			[extHostProtocol.ISuggestDataDtoField.additionalTextEdits]: item.additionalTextEdits && item.additionalTextEdits.map(typeConvert.TextEdit.from),