diff --git a/src/nvim/mapping.c b/src/nvim/mapping.c
index fd97bfc18b..a28bb2473b 100644
--- a/src/nvim/mapping.c
+++ b/src/nvim/mapping.c
@@ -2578,21 +2578,23 @@ const char *did_set_langmap(optset_T *args)
         p++;
       }
       int from = utf_ptr2char(p);
+      const char *const from_ptr = p;
       int to = NUL;
+      const char *to_ptr = "";
       if (p2 == NULL) {
         MB_PTR_ADV(p);
         if (p[0] != ',') {
           if (p[0] == '\\') {
             p++;
           }
-          to = utf_ptr2char(p);
+          to = utf_ptr2char(to_ptr = p);
         }
       } else {
         if (p2[0] != ',') {
           if (p2[0] == '\\') {
             p2++;
           }
-          to = utf_ptr2char(p2);
+          to = utf_ptr2char(to_ptr = p2);
         }
       }
       if (to == NUL) {
@@ -2605,7 +2607,10 @@ const char *did_set_langmap(optset_T *args)
       if (from >= 256) {
         langmap_set_entry(from, to);
       } else {
-        assert(to <= UCHAR_MAX);
+        if (to > UCHAR_MAX) {
+          swmsg(true, "'langmap': mapping from %.*s to %.*s will not work properly",
+                utf_ptr2len(from_ptr), from_ptr, utf_ptr2len(to_ptr), to_ptr);
+        }
         langmap_mapchar[from & 255] = (uint8_t)to;
       }
 
diff --git a/test/functional/editor/langmap_spec.lua b/test/functional/editor/langmap_spec.lua
index e50e19a468..3ea67b76e2 100644
--- a/test/functional/editor/langmap_spec.lua
+++ b/test/functional/editor/langmap_spec.lua
@@ -266,6 +266,18 @@ describe("'langmap'", function()
     command('nnoremap ï ix<esc>')
     testrecording('x', 'xhello', local_setup)
   end)
+  it('does not crash when mapping char < 256 to char >= 256', function()
+    -- Note: this warning should be removed when the pending tests above are fixed.
+    local wmsg = table.concat({
+      "'langmap': mapping from e to ε will not work properly",
+      "'langmap': mapping from ü to μ will not work properly",
+    }, '\n')
+    -- ä ë ï ö ü ÿ < 256, β γ ε μ >= 256
+    command('set langmap=iw,wi,äë,ëä,ïx,xï,βγ,γβ,eε,εe,üμ,μü,ÿy,yÿ')
+    eq(wmsg, n.exec_capture('messages'))
+    command('messages clear | set langmap=iwäëïxβγeεüμÿy;wiëäxïγβεeμüyÿ')
+    eq(wmsg, n.exec_capture('messages'))
+  end)
   -- This test is to ensure the behaviour doesn't change from what's already
   -- around. I (hardenedapple) personally think this behaviour should be
   -- changed.