diff --git a/src/formdata/FormData.php b/src/formdata/FormData.php
index 006f2a0..c47c957 100644
--- a/src/formdata/FormData.php
+++ b/src/formdata/FormData.php
@@ -47,7 +47,7 @@ public function value(string $key): string {
         $lookupKey = $this->translateKey($key);
 
         if (!$this->has($lookupKey)) {
-            throw new FormDataException(sprintf('No such key: %s', $key));
+            throw new FormDataException(sprintf('No such key: %s (%s)', $key, $lookupKey));
         }
 
         return $this->values[$lookupKey];
@@ -72,6 +72,12 @@ private function flattenArray(array $values, array $keyPrefixes = []): void {
     }
 
     private function translateKey(string $key): string {
+        if (preg_match('/^[^\p{C}\[\]]+(?:\[[^\p{C}\[\]]*\])*$/u', $key) === 0) {
+            throw new FormDataException(
+                sprintf('Invalid key name syntax ("%s") - cannot translate', $key)
+            );
+        }
+
         return implode('|', preg_split('/\[|\]/', $key, flags: PREG_SPLIT_NO_EMPTY));
     }
 }
diff --git a/tests/formdata/FormDataTest.php b/tests/formdata/FormDataTest.php
index 26f8e63..d216a73 100644
--- a/tests/formdata/FormDataTest.php
+++ b/tests/formdata/FormDataTest.php
@@ -1,17 +1,15 @@
 <?php declare(strict_types = 1);
 namespace Templado\Engine;
 
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 use stdClass;
 
-/**
- * @covers \Templado\Engine\FormData
- */
+#[CoversClass(FormData::class)]
 class FormDataTest extends TestCase {
 
-    /**
-     * @dataProvider validDataProvider
-     */
+    #[DataProvider('validDataProvider')]
     public function testCanBeInstantiatedWithUsableData(array $data, string $key): void {
         $formdata = new FormData('foo', $data);
         $this->assertInstanceOf(FormData::class, $formdata);
@@ -69,4 +67,37 @@ public function testValueOfExisingKeyCanBeRetrieved(): void {
         $formdata = new FormData('test', ['a' => 'value']);
         $this->assertEquals('value', $formdata->value('a'));
     }
+
+    public function testHandlesMixedNestedArrayTypes(): void {
+        $formdata = new FormData('test', ['a' => 'a', 'b' => ['c','d'], 'e' => ['f' => ['g' => 'h']]]);
+        $this->assertEquals('a', $formdata->value('a'));
+        $this->assertEquals('c', $formdata->value('b[0]'));
+        $this->assertEquals('d', $formdata->value('b[1]'));
+        $this->assertEquals('h', $formdata->value('e[f][g]'));
+    }
+
+    #[DataProvider('invalidNameProvider')]
+    public function testThrowsExceptionOnInvalidNameSyntax(string $input): void {
+        $this->expectException(FormDataException::class);
+        (new FormData('foo', []))->has($input);
+    }
+
+    public static function invalidNameProvider(): array {
+        return [
+            'foo[]0' => ['foo[]0'],
+            'foo[0]a' => ['foo[0]a'],
+            'foo[0]a[1]' => ['foo[0]a[1]'],
+            'foo[' => ['foo['],
+            'foo]' => ['foo]'],
+            'foo[a[]]' => ['foo[a[]]'],
+            '[]foo' => ['[]foo'],
+        ];
+    }
+
+    public function testAcceptsValidNames(): void {
+        $this->assertFalse(
+            (new FormData('foo', []))->has("😀")
+        );
+    }
+
 }