diff --git a/src/Assert.php b/src/Assert.php
index 91afaa0b..09e69ab1 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -1045,6 +1045,8 @@ public static function startsWith($value, $prefix, $message = '')
     }
 
     /**
+     * @psalm-assert string $value
+     *
      * @param mixed  $value
      * @param string $message
      *
@@ -1052,6 +1054,8 @@ public static function startsWith($value, $prefix, $message = '')
      */
     public static function startsWithLetter($value, $message = '')
     {
+        static::string($value);
+
         $valid = isset($value[0]);
 
         if ($valid) {
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index 4a23b061..4546dab5 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -265,11 +265,14 @@ public function getTests()
             array('startsWith', array('😄😑☹️', '😑'), false),
             array('startsWith', array('', '😑'), false),
             array('startsWithLetter', array('abcd'), true),
+            array('startsWithLetter', array(array(66)), false),
             array('startsWithLetter', array('a'), true),
             array('startsWithLetter', array('a1'), true),
             array('startsWithLetter', array('1abcd'), false),
             array('startsWithLetter', array('1'), false),
             array('startsWithLetter', array(''), false),
+            array('startsWithLetter', array(null), false),
+            array('startsWithLetter', array(66), false),
             array('endsWith', array('abcd', 'cd'), true),
             array('endsWith', array('abcd', 'bc'), false),
             array('endsWith', array('', 'bc'), false),
diff --git a/tests/static-analysis/assert-startsWithLetter.php b/tests/static-analysis/assert-startsWithLetter.php
new file mode 100644
index 00000000..78defd19
--- /dev/null
+++ b/tests/static-analysis/assert-startsWithLetter.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace Webmozart\Assert\Tests\StaticAnalysis;
+
+use Webmozart\Assert\Assert;
+
+/**
+ * @param mixed $value
+ * @return string
+ */
+function consume($value) : string
+{
+    Assert::startsWithLetter($value);
+    return $value;
+}