diff --git a/src/errors.h b/src/errors.h
index 753e2fe..cde9846 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -686,3 +686,5 @@
 	INIT(= N_("E1244: Bad color string: %s"));
 EXTERN char e_cannot_expand_sfile_in_vim9_function[]
 	INIT(= N_("E1245: Cannot expand <sfile> in a Vim9 function"));
+EXTERN char e_cannot_find_variable_to_unlock_str[]
+	INIT(= N_("E1246: Cannot find variable to (un)lock: %s"));
diff --git a/src/evalvars.c b/src/evalvars.c
index b560734..b1d7b78 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1827,7 +1827,12 @@
 	    // Normal name or expanded name.
 	    di = find_var(lp->ll_name, NULL, TRUE);
 	    if (di == NULL)
+	    {
+		if (in_vim9script())
+		    semsg(_(e_cannot_find_variable_to_unlock_str),
+								  lp->ll_name);
 		ret = FAIL;
+	    }
 	    else if ((di->di_flags & DI_FLAGS_FIX)
 			    && di->di_tv.v_type != VAR_DICT
 			    && di->di_tv.v_type != VAR_LIST)
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index 31f20cb..16f5d55 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1370,6 +1370,23 @@
       unlockvar theList
   END
   CheckDefFailure(lines, 'E1178', 2)
+
+  lines =<< trim END
+      vim9script
+      var name = 'john'
+      lockvar nameX
+  END
+  CheckScriptFailure(lines, 'E1246', 3)
+
+  lines =<< trim END
+      vim9script
+      var name = 'john'
+      def LockIt()
+        lockvar nameX
+      enddef
+      LockIt()
+  END
+  CheckScriptFailure(lines, 'E1246', 1)
 enddef
 
 def Test_substitute_expr()
diff --git a/src/version.c b/src/version.c
index 121ce6e..f2bcb19 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3651,
+/**/
     3650,
 /**/
     3649,