diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index a74fee8ef20..1bea370566c 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4226,7 +4226,7 @@ namespace ts {
         /* @internal */ isTypeAssignableTo(source: Type, target: Type): boolean;
         /* @internal */ createAnonymousType(symbol: Symbol | undefined, members: SymbolTable, callSignatures: Signature[], constructSignatures: Signature[], stringIndexInfo: IndexInfo | undefined, numberIndexInfo: IndexInfo | undefined): Type;
         /* @internal */ createSignature(
-            declaration: SignatureDeclaration,
+            declaration: SignatureDeclaration | undefined,
             typeParameters: readonly TypeParameter[] | undefined,
             thisParameter: Symbol | undefined,
             parameters: readonly Symbol[],
diff --git a/src/services/codefixes/inferFromUsage.ts b/src/services/codefixes/inferFromUsage.ts
index adec45a33ab..f9eaefcebab 100644
--- a/src/services/codefixes/inferFromUsage.ts
+++ b/src/services/codefixes/inferFromUsage.ts
@@ -1135,8 +1135,7 @@ namespace ts.codefix {
                 parameters.push(symbol);
             }
             const returnType = combineFromUsage(combineUsages(calls.map(call => call.return_)));
-            // TODO: GH#18217
-            return checker.createSignature(/*declaration*/ undefined!, /*typeParameters*/ undefined, /*thisParameter*/ undefined, parameters, returnType, /*typePredicate*/ undefined, length, SignatureFlags.None);
+            return checker.createSignature(/*declaration*/ undefined, /*typeParameters*/ undefined, /*thisParameter*/ undefined, parameters, returnType, /*typePredicate*/ undefined, length, SignatureFlags.None);
         }
 
         function addCandidateType(usage: Usage, type: Type | undefined) {