diff --git a/features/scaffold-block.feature b/features/scaffold-block.feature
index 27874f47..868be36e 100644
--- a/features/scaffold-block.feature
+++ b/features/scaffold-block.feature
@@ -31,7 +31,7 @@ Feature: WordPress block code scaffolding
       """
 
   Scenario: Scaffold a block for an invalid plugin slug
-    When I run `wp scaffold plugin plugin.name.with.dots`
+    When I try `wp scaffold plugin plugin.name.with.dots`
     And I try `wp scaffold block some-block --plugin=plugin.name.with.dots`
     Then STDERR should contain:
       """
diff --git a/features/scaffold-plugin-tests.feature b/features/scaffold-plugin-tests.feature
index 95fee4ca..2ab4738e 100644
--- a/features/scaffold-plugin-tests.feature
+++ b/features/scaffold-plugin-tests.feature
@@ -236,7 +236,28 @@ Feature: Scaffold plugin unit tests
     When I try `wp scaffold plugin-tests ../`
     Then STDERR should be:
       """
-      Error: Invalid plugin slug specified. The target directory '{RUN_DIR}/wp-content/plugins/../' is not in '{RUN_DIR}/wp-content/plugins'.
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold plugin-tests my-plugin/`
+    Then STDERR should be:
+      """
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold plugin-tests my-plugin\\`
+    Then STDERR should be:
+      """
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold plugin-tests my_plugin`
+    Then STDERR should be:
+      """
+      Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.
       """
     And the return code should be 1
 
diff --git a/features/scaffold-theme-tests.feature b/features/scaffold-theme-tests.feature
index 7a4f2ac5..a55b06a6 100644
--- a/features/scaffold-theme-tests.feature
+++ b/features/scaffold-theme-tests.feature
@@ -215,7 +215,28 @@ Feature: Scaffold theme unit tests
     When I try `wp scaffold theme-tests ../`
     Then STDERR should be:
       """
-      Error: Invalid theme slug specified. The target directory '{RUN_DIR}/wp-content/themes/../' is not in '{RUN_DIR}/wp-content/themes'.
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold theme-tests t12child/`
+    Then STDERR should be:
+      """
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold theme-tests t12child\\`
+    Then STDERR should be:
+      """
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
+      """
+    And the return code should be 1
+
+    When I try `wp scaffold theme-tests t12_child`
+    Then STDERR should be:
+      """
+      Error: Invalid theme slug specified. The slug can only contain alphanumeric characters and dashes.
       """
     And the return code should be 1
 
diff --git a/src/Scaffold_Command.php b/src/Scaffold_Command.php
index e92a032e..3234f70b 100644
--- a/src/Scaffold_Command.php
+++ b/src/Scaffold_Command.php
@@ -829,6 +829,10 @@ private function scaffold_plugin_theme_tests( $args, $assoc_args, $type ) {
 			if ( in_array( $slug, [ '.', '..' ], true ) ) {
 				WP_CLI::error( "Invalid {$type} slug specified. The slug cannot be '.' or '..'." );
 			}
+			// Validate slug contains only alphanumeric characters and dashes.
+			if ( ! preg_match( '/^[a-zA-Z0-9-]+$/', $slug ) ) {
+				WP_CLI::error( "Invalid {$type} slug specified. The slug can only contain alphanumeric characters and dashes." );
+			}
 			if ( 'theme' === $type ) {
 				$theme = wp_get_theme( $slug );
 				if ( $theme->exists() ) {