diff --git a/invokeai/frontend/web/src/features/nodes/store/util/validateConnection.ts b/invokeai/frontend/web/src/features/nodes/store/util/validateConnection.ts
index debf294557..b6b5a43d37 100644
--- a/invokeai/frontend/web/src/features/nodes/store/util/validateConnection.ts
+++ b/invokeai/frontend/web/src/features/nodes/store/util/validateConnection.ts
@@ -6,13 +6,19 @@ import { validateConnectionTypes } from 'features/nodes/store/util/validateConne
 import type { AnyNode } from 'features/nodes/types/invocation';
 import type { Connection as NullableConnection, Edge } from 'reactflow';
 import type { O } from 'ts-toolbelt';
+import { assert } from 'tsafe';
 
 type Connection = O.NonNullable<NullableConnection>;
 
-export type ValidateConnectionResult = {
-  isValid: boolean;
-  messageTKey?: string;
-};
+export type ValidateConnectionResult =
+  | {
+      isValid: true;
+      messageTKey?: string;
+    }
+  | {
+      isValid: false;
+      messageTKey: string;
+    };
 
 export type ValidateConnectionFunc = (
   connection: Connection,
@@ -22,10 +28,20 @@ export type ValidateConnectionFunc = (
   ignoreEdge: Edge | null
 ) => ValidateConnectionResult;
 
-export const buildResult = (isValid: boolean, messageTKey?: string): ValidateConnectionResult => ({
-  isValid,
-  messageTKey,
-});
+export const buildResult = (isValid: boolean, messageTKey?: string): ValidateConnectionResult => {
+  if (isValid) {
+    return {
+      isValid,
+      messageTKey,
+    };
+  } else {
+    assert(messageTKey !== undefined);
+    return {
+      isValid,
+      messageTKey,
+    };
+  }
+};
 
 const getEqualityPredicate =
   (c: Connection) =>