diff --git a/src/librustc_target/abi/mod.rs b/src/librustc_target/abi/mod.rs
index 26d37f196bef..fde5c5bed4d9 100644
--- a/src/librustc_target/abi/mod.rs
+++ b/src/librustc_target/abi/mod.rs
@@ -738,7 +738,11 @@ impl FieldPlacement {
 
     pub fn offset(&self, i: usize) -> Size {
         match *self {
-            FieldPlacement::Union(_) => Size::ZERO,
+            FieldPlacement::Union(count) => {
+                assert!(i < count,
+                        "Tried to access field {} of union with {} fields", i, count);
+                Size::ZERO
+            },
             FieldPlacement::Array { stride, count } => {
                 let i = i as u64;
                 assert!(i < count);