diff --git a/test/acm_random.h b/test/acm_random.h
index ff5c93e..a29ced2 100644
--- a/test/acm_random.h
+++ b/test/acm_random.h
@@ -32,6 +32,12 @@
     return (value >> 15) & 0xffff;
   }
 
+  int16_t Rand9Signed(void) {
+    // Use 9 bits: values between 255 (0x0FF) and -256 (0x100).
+    const uint32_t value = random_.Generate(512);
+    return static_cast<int16_t>(value - 256);
+  }
+
   uint8_t Rand8(void) {
     const uint32_t value =
         random_.Generate(testing::internal::Random::kMaxRange);