diff --git a/plantuml-mode.el b/plantuml-mode.el
index a94426d..097e208 100644
--- a/plantuml-mode.el
+++ b/plantuml-mode.el
@@ -161,18 +161,20 @@
 (defvar-local plantuml-exec-mode plantuml-default-exec-mode
   "The Plantuml execution mode. See `plantuml-default-exec-mode' for acceptable values.")
 
-(defun plantuml-set-exec-mode ()
-  "Set the execution mode for PlantUML."
-  (interactive)
-  (let* ((completion-ignore-case t)
-         (supported-modes        '("jar" "server")))
-    (setq plantuml-exec-mode (intern (completing-read (format "Exec mode [%s]: " plantuml-exec-mode)
-                                                      supported-modes
-                                                      nil
-                                                      t
-                                                      nil
-                                                      nil
-                                                      plantuml-exec-mode)))))
+(defun plantuml-set-exec-mode (mode)
+  "Set the execution mode MODE for PlantUML."
+  (interactive (let* ((completion-ignore-case t)
+                      (supported-modes        '("jar" "server")))
+                 (completing-read (format "Exec mode [%s]: " plantuml-exec-mode)
+                                  supported-modes
+                                  nil
+                                  t
+                                  nil
+                                  nil
+                                  plantuml-exec-mode)))
+  (if (member mode '("jar" "server"))
+      (setq plantuml-exec-mode (intern mode))
+    (error (concat "Unsupported mode:" mode))))
 
 (defun plantuml-enable-debug ()
   "Enables debug messages into the *PLANTUML Messages* buffer."
diff --git a/test/plantuml-config-test.el b/test/plantuml-config-test.el
new file mode 100644
index 0000000..8fc4a2a
--- /dev/null
+++ b/test/plantuml-config-test.el
@@ -0,0 +1,33 @@
+;;; plantuml-config-test.el --- tests for plantuml-mode configuration knobs  -*- lexical-binding: t; -*-
+
+;; Author: Carlo Sciolla
+;; Maintainer: Carlo Sciolla
+;; URL: https://github.com/skuro/plantuml-mode
+
+;;; Commentary:
+
+;; Test user-accessible configuration knobs
+
+;;; Code:
+
+(require 'plantuml-mode)
+
+(ert-deftest plantuml-config-test/set-exec-mode-happy-path ()
+  "Test switching execution modes"
+  (let ((orig-mode plantuml-exec-mode))
+    ;; happy flows:
+    (plantuml-set-exec-mode "server")
+    (should (equal 'server plantuml-exec-mode))
+    (plantuml-set-exec-mode "jar")
+    (should (equal 'jar plantuml-exec-mode))
+
+    (setq plantuml-exec-mode orig-mode)))
+
+(ert-deftest plantuml-config-test/set-exec-mode-wrong-mode ()
+  "Test setting the exec mode with the wrong text"
+  :expected-result :failed
+  (plantuml-set-exec-mode "turing-machine"))
+
+(provide 'plantuml-mode-config-test)
+
+;;; plantuml-config-test.el ends here