diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 64ef1b552f5fa..87827d0c14ae4 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -416,8 +416,8 @@ namespace Harness.LanguageService {
         getCompletionsAtPosition(fileName: string, position: number): ts.CompletionInfo {
             return unwrapJSONCallResult(this.shim.getCompletionsAtPosition(fileName, position));
         }
-        getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: ts.FormatCodeOptions): ts.CompletionEntryDetails {
-            return unwrapJSONCallResult(this.shim.getCompletionEntryDetails(fileName, position, entryName, JSON.stringify(options)));
+        getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: ts.FormatCodeOptions | undefined, source: string | undefined): ts.CompletionEntryDetails {
+            return unwrapJSONCallResult(this.shim.getCompletionEntryDetails(fileName, position, entryName, JSON.stringify(options), source));
         }
         getCompletionEntrySymbol(): ts.Symbol {
             throw new Error("getCompletionEntrySymbol not implemented across the shim layer.");
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 6e979e22023fc..3e0a1e75f4183 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -141,7 +141,7 @@ namespace ts {
         getEncodedSemanticClassifications(fileName: string, start: number, length: number): string;
 
         getCompletionsAtPosition(fileName: string, position: number): string;
-        getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: string/*Services.FormatCodeOptions*/): string;
+        getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: string/*Services.FormatCodeOptions*/, source: string | undefined): string;
 
         getQuickInfoAtPosition(fileName: string, position: number): string;
 
@@ -893,12 +893,12 @@ namespace ts {
         }
 
         /** Get a string based representation of a completion list entry details */
-        public getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: string/*Services.FormatCodeOptions*/) {
+        public getCompletionEntryDetails(fileName: string, position: number, entryName: string, options: string/*Services.FormatCodeOptions*/, source: string | undefined) {
             return this.forwardJSONCall(
                 `getCompletionEntryDetails('${fileName}', ${position}, '${entryName}')`,
                 () => {
                     const localOptions: ts.FormatCodeOptions = JSON.parse(options);
-                    return this.languageService.getCompletionEntryDetails(fileName, position, entryName, localOptions);
+                    return this.languageService.getCompletionEntryDetails(fileName, position, entryName, localOptions, source);
                 }
             );
         }
diff --git a/src/services/types.ts b/src/services/types.ts
index c854eb8aaccfc..e1803af512a18 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -238,8 +238,15 @@ namespace ts {
 
         getCompletionsAtPosition(fileName: string, position: number): CompletionInfo;
         // "options" and "source" are optional only for backwards-compatibility
-        getCompletionEntryDetails(fileName: string, position: number, name: string, options?: FormatCodeOptions | FormatCodeSettings, source?: string): CompletionEntryDetails;
-        getCompletionEntrySymbol(fileName: string, position: number, name: string, source?: string): Symbol;
+        getCompletionEntryDetails(
+            fileName: string,
+            position: number,
+            name: string,
+            // These are potentially undefined only for backwards-compatibility.
+            options: FormatCodeOptions | FormatCodeSettings | undefined,
+            source: string | undefined,
+        ): CompletionEntryDetails;
+        getCompletionEntrySymbol(fileName: string, position: number, name: string, source: string | undefined): Symbol;
 
         getQuickInfoAtPosition(fileName: string, position: number): QuickInfo;
 
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 5d439e840b09f..1ef983e353ba4 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -3915,8 +3915,8 @@ declare namespace ts {
         getEncodedSyntacticClassifications(fileName: string, span: TextSpan): Classifications;
         getEncodedSemanticClassifications(fileName: string, span: TextSpan): Classifications;
         getCompletionsAtPosition(fileName: string, position: number): CompletionInfo;
-        getCompletionEntryDetails(fileName: string, position: number, name: string, options?: FormatCodeOptions | FormatCodeSettings, source?: string): CompletionEntryDetails;
-        getCompletionEntrySymbol(fileName: string, position: number, name: string, source?: string): Symbol;
+        getCompletionEntryDetails(fileName: string, position: number, name: string, options: FormatCodeOptions | FormatCodeSettings | undefined, source: string | undefined): CompletionEntryDetails;
+        getCompletionEntrySymbol(fileName: string, position: number, name: string, source: string | undefined): Symbol;
         getQuickInfoAtPosition(fileName: string, position: number): QuickInfo;
         getNameOrDottedNameSpan(fileName: string, startPos: number, endPos: number): TextSpan;
         getBreakpointStatementAtPosition(fileName: string, position: number): TextSpan;
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 2afef7d692cd9..a82fc00d9d20f 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -3915,8 +3915,8 @@ declare namespace ts {
         getEncodedSyntacticClassifications(fileName: string, span: TextSpan): Classifications;
         getEncodedSemanticClassifications(fileName: string, span: TextSpan): Classifications;
         getCompletionsAtPosition(fileName: string, position: number): CompletionInfo;
-        getCompletionEntryDetails(fileName: string, position: number, name: string, options?: FormatCodeOptions | FormatCodeSettings, source?: string): CompletionEntryDetails;
-        getCompletionEntrySymbol(fileName: string, position: number, name: string, source?: string): Symbol;
+        getCompletionEntryDetails(fileName: string, position: number, name: string, options: FormatCodeOptions | FormatCodeSettings | undefined, source: string | undefined): CompletionEntryDetails;
+        getCompletionEntrySymbol(fileName: string, position: number, name: string, source: string | undefined): Symbol;
         getQuickInfoAtPosition(fileName: string, position: number): QuickInfo;
         getNameOrDottedNameSpan(fileName: string, startPos: number, endPos: number): TextSpan;
         getBreakpointStatementAtPosition(fileName: string, position: number): TextSpan;