Fix potential NPE if we try to evaluate isMissingIOWrapper() inside IO(). (#479)

1 files changed, 3 insertions, 1 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
index 71c441a7..a857ae85 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
@@ -155,7 +155,9 @@ object Binding {
             //  we know the wrapper is missing, whether or not the element is a member of io.
             //  But if it's not an io element, we want to issue the complementary "unbound" error.
             //  Revisit this when we collect error messages instead of throwing exceptions.
-            x.io.flatten.contains(element)
+            // The null test below is due to the fact that we may be evaluating the arguments
+            //  of the IO() wrapper itself.
+            (x.io != null) && x.io.flatten.contains(element)
           }
         }
       }