diff --git a/test/_global/mermaid_invalid_user.sh b/test/_global/mermaid_invalid_user.sh
new file mode 100755
index 0000000..99dd7be
--- /dev/null
+++ b/test/_global/mermaid_invalid_user.sh
@@ -0,0 +1,18 @@
+#!/bin/bash
+set -e
+
+# SCRIPT_DIR is the directory of this script
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+INSTALL_SCRIPT="$SCRIPT_DIR/../../src/mermaid/install.sh"
+
+echo "Testing invalid USERNAME..."
+output=$(USERNAME="-invalid" bash "$INSTALL_SCRIPT" 2>&1 || true)
+
+if echo "$output" | grep -q "Error: Invalid USERNAME"; then
+    echo "✅ Success: Caught invalid username"
+    exit 0
+else
+    echo "❌ Fail: Did not catch invalid username"
+    echo "Output was: $output"
+    exit 1
+fi