diff --git a/src/services/completions.ts b/src/services/completions.ts
index 535f6f4f745..d8bd2c705e8 100644
--- a/src/services/completions.ts
+++ b/src/services/completions.ts
@@ -1653,8 +1653,8 @@ namespace ts.Completions {
             if (!(classElementModifierFlags & ModifierFlags.Private)) {
                 // List of property symbols of base type that are not private and already implemented
                 const baseSymbols = flatMap(getAllSuperTypeNodes(decl), baseTypeNode => {
-                    const type = typeChecker.getTypeAtLocation(baseTypeNode)!; // TODO: GH#18217
-                    return typeChecker.getPropertiesOfType(classElementModifierFlags & ModifierFlags.Static ? typeChecker.getTypeOfSymbolAtLocation(type.symbol, decl) : type);
+                    const type = typeChecker.getTypeAtLocation(baseTypeNode);
+                    return type && typeChecker.getPropertiesOfType(classElementModifierFlags & ModifierFlags.Static ? typeChecker.getTypeOfSymbolAtLocation(type.symbol, decl) : type);
                 });
                 symbols = filterClassMembersList(baseSymbols, decl.members, classElementModifierFlags);
             }
diff --git a/tests/cases/fourslash/completionListClassMembersWithSuperClassFromUnknownNamespace.ts b/tests/cases/fourslash/completionListClassMembersWithSuperClassFromUnknownNamespace.ts
new file mode 100644
index 00000000000..8e6aab7394b
--- /dev/null
+++ b/tests/cases/fourslash/completionListClassMembersWithSuperClassFromUnknownNamespace.ts
@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////class Child extends Namespace.Parent {
+////    /**/
+////}
+
+goTo.marker("");
+verify.completionListContainsClassElementKeywords();
+verify.completionListCount(verify.allowedClassElementKeywords.length);
\ No newline at end of file