diff --git a/src/SourceLocator/SourceStubber/ReflectionSourceStubber.php b/src/SourceLocator/SourceStubber/ReflectionSourceStubber.php
index b0bf3580e..98acec683 100644
--- a/src/SourceLocator/SourceStubber/ReflectionSourceStubber.php
+++ b/src/SourceLocator/SourceStubber/ReflectionSourceStubber.php
@@ -593,7 +593,11 @@ private function setParameterDefaultValue(ReflectionParameter $parameterReflecti
         }
 
         if (! $parameterReflection->isDefaultValueAvailable()) {
-            $parameterNode->setDefault(null);
+            if ($parameterReflection->allowsNull()) {
+                $parameterNode->setDefault(null);
+            } else {
+                $parameterNode->setDefault(new Node\Expr\ConstFetch(new FullyQualified('UNKNOWN')));
+            }
 
             return;
         }