diff --git a/core/wasm_shims/tooldata/tooldata_probe_main.cc b/core/wasm_shims/tooldata/tooldata_probe_main.cc
index 9c8da76..d5a61b4 100644
--- a/core/wasm_shims/tooldata/tooldata_probe_main.cc
+++ b/core/wasm_shims/tooldata/tooldata_probe_main.cc
@@ -272,6 +272,23 @@ int main() {
     if (tooldata_put(tool, 12) != IDX_NEW) {
         return 2;
     }
+    CANON_TOOL_TABLE lower_index_tool = tooldata_entry_init();
+    lower_index_tool.toolno = 3;
+    lower_index_tool.pocketno = 5;
+    // LinuxCNC src/emc/tooldata/tooldata_mmap.cc returns IDX_OK for writes at
+    // or below the current last_index and leaves last_index unchanged.
+    if (tooldata_put(lower_index_tool, 5) != IDX_OK) {
+        return 80;
+    }
+    if (tooldata_last_index_get() != 12) {
+        return 81;
+    }
+    CANON_TOOL_TABLE lower_index_loaded = tooldata_entry_init();
+    if (tooldata_get(&lower_index_loaded, 5) != IDX_OK ||
+        lower_index_loaded.toolno != 3 ||
+        lower_index_loaded.pocketno != 5) {
+        return 82;
+    }
 
     CANON_TOOL_TABLE loaded = tooldata_entry_init();
     if (tooldata_get(&loaded, 12) != IDX_OK) {