diff --git a/src/SerializationTrait.php b/src/SerializationTrait.php
index 0bebc6c..331e5bf 100644
--- a/src/SerializationTrait.php
+++ b/src/SerializationTrait.php
@@ -50,12 +50,21 @@ protected function serializeValue(mixed $value): string
     /**
      * Deserialize a value retrieved from a string-only backend.
      *
-     * @param string $stored  The raw stored string.
+     * Accepts mixed so co-tenant writers (legacy Horde_HashTable_Memcache,
+     * prior releases of this package, or unrelated components sharing the
+     * same memcached pool) that stored bare non-string scalars are passed
+     * through instead of tripping a strict-type error. String values still
+     * go through the s:/p: prefix protocol; anything else is returned as-is.
+     *
+     * @param mixed $stored  The raw stored value as returned by the backend.
      *
      * @return mixed  The original value.
      */
-    protected function deserializeValue(string $stored): mixed
+    protected function deserializeValue(mixed $stored): mixed
     {
+        if (!is_string($stored)) {
+            return $stored;
+        }
         if (str_starts_with($stored, 's:')) {
             return substr($stored, 2);
         }
diff --git a/test/src/Unit/SerializationTraitTest.php b/test/src/Unit/SerializationTraitTest.php
new file mode 100644
index 0000000..190cc37
--- /dev/null
+++ b/test/src/Unit/SerializationTraitTest.php
@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Horde\HashTable\Test\Src\Unit;
+
+use Horde\HashTable\SerializationTrait;
+use PHPUnit\Framework\Attributes\CoversTrait;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Exposes the protected trait methods so tests can hit them directly.
+ */
+final class SerializationTraitConsumer
+{
+    use SerializationTrait;
+
+    public function encode(mixed $value): string
+    {
+        return $this->serializeValue($value);
+    }
+
+    public function decode(mixed $stored): mixed
+    {
+        return $this->deserializeValue($stored);
+    }
+}
+
+#[CoversTrait(SerializationTrait::class)]
+final class SerializationTraitTest extends TestCase
+{
+    private SerializationTraitConsumer $sut;
+
+    protected function setUp(): void
+    {
+        $this->sut = new SerializationTraitConsumer();
+    }
+
+    #[Test]
+    public function stringRoundTrip(): void
+    {
+        $this->assertSame('hello', $this->sut->decode($this->sut->encode('hello')));
+    }
+
+    #[Test]
+    public function intRoundTrip(): void
+    {
+        $this->assertSame(42, $this->sut->decode($this->sut->encode(42)));
+    }
+
+    #[Test]
+    public function arrayRoundTrip(): void
+    {
+        $value = ['a' => 1, 'b' => [2, 3]];
+        $this->assertSame($value, $this->sut->decode($this->sut->encode($value)));
+    }
+
+    /**
+     * Values retrieved from the backend that never went through the trait's
+     * writer (co-tenant writer, legacy driver, prior release) must pass
+     * through instead of tripping a strict-type error.
+     */
+    #[Test]
+    public function nonStringInputPassesThroughUnchanged(): void
+    {
+        $this->assertSame(1735689600, $this->sut->decode(1735689600));
+        $this->assertSame(true, $this->sut->decode(true));
+        $this->assertSame(null, $this->sut->decode(null));
+        $this->assertSame(['x' => 1], $this->sut->decode(['x' => 1]));
+    }
+
+    /**
+     * Bare (unprefixed) strings — the "legacy data" case documented in the
+     * trait — must round-trip as a raw string.
+     */
+    #[Test]
+    public function unprefixedStringTreatedAsRaw(): void
+    {
+        $this->assertSame('legacy', $this->sut->decode('legacy'));
+    }
+
+    #[Test]
+    public function stringPrefixShortForm(): void
+    {
+        // Bytes that happen to look like the p: prefix don't get confused
+        // with prefixed content when they came from the s: writer.
+        $encoded = $this->sut->encode('p:not-really-serialized');
+        $this->assertSame('p:not-really-serialized', $this->sut->decode($encoded));
+    }
+}