diff --git a/build/gen_stub.php b/build/gen_stub.php
index dc762f8d6cb..3bf9fb995dc 100755
--- a/build/gen_stub.php
+++ b/build/gen_stub.php
@@ -394,6 +394,10 @@ class SimpleType {
         return $this->isBuiltin && $this->name === 'array';
     }
 
+    public function isMixed(): bool {
+        return $this->isBuiltin && $this->name === 'mixed';
+    }
+
     public function toTypeCode(): string {
         assert($this->isBuiltin);
         switch ($this->name) {
@@ -4233,7 +4237,7 @@ function parseFunctionLike(
                 $type && !$type->isNullable()
             ) {
                 $simpleType = $type->tryToSimpleType();
-                if ($simpleType === null) {
+                if ($simpleType === null || !$simpleType->isMixed()) {
                     throw new Exception("Parameter $varName has null default, but is not nullable");
                 }
             }
@@ -4342,13 +4346,26 @@ function parseConstLike(
         throw new Exception("Missing type for constant " . $name->__toString());
     }
 
+    $constType = $type ? Type::fromNode($type) : null;
+    $constPhpDocType = $phpDocType ? Type::fromString($phpDocType) : null;
+
+    if ($const->value instanceof Expr\ConstFetch &&
+        $const->value->name->toLowerString() === "null" &&
+        $constType && !$constType->isNullable()
+    ) {
+        $simpleType = $constType->tryToSimpleType();
+        if ($simpleType === null || !$simpleType->isMixed()) {
+            throw new Exception("Constant " . $name->__toString() . " has null value, but is not nullable");
+        }
+    }
+
     return new ConstInfo(
         $name,
         $flags,
         $const->value,
         $prettyPrinter->prettyPrintExpr($const->value),
-        $type ? Type::fromNode($type) : null,
-        $phpDocType ? Type::fromString($phpDocType) : null,
+        $constType,
+        $constPhpDocType,
         $deprecated,
         $cond,
         $cValue,
@@ -4401,9 +4418,8 @@ function parseProperty(
         $propertyType && !$propertyType->isNullable()
     ) {
         $simpleType = $propertyType->tryToSimpleType();
-        if ($simpleType === null) {
-            throw new Exception(
-                "Property $class::\$$property->name has null default, but is not nullable");
+        if ($simpleType === null || !$simpleType->isMixed()) {
+            throw new Exception("Property $class::\$$property->name has null default, but is not nullable");
         }
     }