diff --git a/actions-languageservice/src/validate-action.ts b/actions-languageservice/src/validate-action.ts
index 8a5d30d..b5c1931 100644
--- a/actions-languageservice/src/validate-action.ts
+++ b/actions-languageservice/src/validate-action.ts
@@ -25,6 +25,11 @@ export async function validateAction(
 
   const actionInputs = await config.getActionInputs(action);
   if (actionInputs === undefined) {
+    diagnostics.push({
+      severity: DiagnosticSeverity.Error,
+      range: mapRange(step.uses.range),
+      message: `Unable to resolve action \`${step.uses.value}\`, repository or version not found`
+    });
     return;
   }
 
diff --git a/actions-languageservice/src/validate.actions.test.ts b/actions-languageservice/src/validate.actions.test.ts
index b9be81b..05842bb 100644
--- a/actions-languageservice/src/validate.actions.test.ts
+++ b/actions-languageservice/src/validate.actions.test.ts
@@ -10,7 +10,7 @@ registerLogger(new TestLogger());
 
 const validationConfig: ValidationConfig = {
   getActionInputs: async (ref: ActionReference) => {
-    let inputs: Record<string, ActionInput> = {};
+    let inputs: Record<string, ActionInput> | undefined = undefined;
     switch (ref.owner + "/" + ref.name + "@" + ref.ref) {
       case "actions/checkout@v3":
         inputs = {
@@ -58,6 +58,7 @@ const validationConfig: ValidationConfig = {
     return inputs;
   }
 };
+
 describe("validate action steps", () => {
   it("valid action reference", async () => {
     const input = `
@@ -73,6 +74,35 @@ jobs:
     expect(result).toEqual([]);
   });
 
+  it("action does not exist", async () => {
+    const input = `
+    on: push
+    jobs:
+      build:
+        runs-on: ubuntu-latest
+        steps:
+        - uses: actions/does-not-exist@v3
+    `;
+    const result = await validate(createDocument("wf.yaml", input), validationConfig);
+
+    expect(result).toEqual([
+      {
+        message: "Unable to resolve action `actions/does-not-exist@v3`, repository or version not found",
+        range: {
+          end: {
+            character: 41,
+            line: 6
+          },
+          start: {
+            character: 16,
+            line: 6
+          }
+        },
+        severity: DiagnosticSeverity.Error
+      }
+    ]);
+  });
+
   it("invalid input", async () => {
     const input = `
 on: push