diff --git a/docs/changes/README.md b/docs/changes/README.md
index ea3078b2c..ee079a86f 100644
--- a/docs/changes/README.md
+++ b/docs/changes/README.md
@@ -3,6 +3,9 @@
 
 ## [Unreleased](https://github.com/GradleUp/shadow/compare/9.0.0...HEAD) - 2025-xx-xx
 
+### Added
+
+- Improve the error message for empty `mainClassName`. ([#1601](https://github.com/GradleUp/shadow/pull/1601))
 
 ## [9.0.0](https://github.com/GradleUp/shadow/compare/9.0.0) - 2025-08-07
 
diff --git a/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/ApplicationPluginTest.kt b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/ApplicationPluginTest.kt
index f931db3b5..ef7d1a14c 100644
--- a/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/ApplicationPluginTest.kt
+++ b/src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/ApplicationPluginTest.kt
@@ -158,6 +158,17 @@ class ApplicationPluginTest : BasePluginTest() {
     assertions(run(":run").output, "bar")
   }
 
+  @Test
+  fun errorWhenMainClassNotSet() {
+    prepare(mainClassBlock = "")
+
+    val result = runWithFailure(runShadowTask)
+
+    assertThat(result.output).contains(
+      "The main class must be specified and not left empty in `application.mainClass` or manifest attributes.",
+    )
+  }
+
   @Test
   fun addExtraFilesIntoDistribution() {
     path("extra/echo.sh").writeText("echo 'Hello, World!'")
@@ -220,6 +231,7 @@ class ApplicationPluginTest : BasePluginTest() {
   private fun prepare(
     mainClassWithImports: Boolean = false,
     projectBlock: String = "",
+    mainClassBlock: String = "mainClass = 'my.Main'",
     applicationBlock: String = "",
     settingsBlock: String = "",
     dependenciesBlock: String = "implementation 'my:a:1.0'",
@@ -231,7 +243,7 @@ class ApplicationPluginTest : BasePluginTest() {
         apply plugin: 'application'
         $projectBlock
         application {
-          mainClass = 'my.Main'
+          $mainClassBlock
           $applicationBlock
         }
         dependencies {
diff --git a/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt b/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt
index e6e30d304..72d362abc 100644
--- a/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt
+++ b/src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt
@@ -129,11 +129,14 @@ public abstract class ShadowApplicationPlugin : Plugin<Project> {
   protected open fun Project.configureShadowJarMainClass() {
     val mainClassName = applicationExtension.mainClass
     tasks.shadowJar.configure { task ->
-      task.inputs.property("mainClassName", mainClassName)
       task.doFirst {
         // Inject the attribute if it is not already present.
         if (!task.manifest.attributes.contains(mainClassAttributeKey)) {
-          task.manifest.attributes[mainClassAttributeKey] = mainClassName.get()
+          task.manifest.attributes[mainClassAttributeKey] = mainClassName.orNull.also { value ->
+            if (value.isNullOrEmpty()) {
+              error("The main class must be specified and not left empty in `application.mainClass` or manifest attributes.")
+            }
+          }
         }
       }
     }