diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7dddeaac507..9a8e9eb1ee8 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -34729,7 +34729,8 @@ namespace ts {
         function getTypeArgumentConstraint(node: TypeNode): Type | undefined {
             const typeReferenceNode = tryCast(node.parent, isTypeReferenceType);
             if (!typeReferenceNode) return undefined;
-            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode)!; // TODO: GH#18217
+            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode);
+            if (!typeParameters) return undefined;
             const constraint = getConstraintOfTypeParameter(typeParameters[typeReferenceNode.typeArguments!.indexOf(node)]);
             return constraint && instantiateType(constraint, createTypeMapper(typeParameters, getEffectiveTypeArguments(typeReferenceNode, typeParameters)));
         }
diff --git a/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts b/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts
new file mode 100644
index 00000000000..7c1a11889a6
--- /dev/null
+++ b/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts
@@ -0,0 +1,10 @@
+/// <reference path="./fourslash.ts" />
+
+////interface Foo {}
+////type Bar = {};
+////
+////let x: Foo<"/*1*/">;
+////let y: Bar<"/*2*/">;
+
+verify.completions({ marker: test.markers() });
+