2 files changed, 7 insertions, 0 deletions
diff --git a/src/s2wasm.h b/src/s2wasm.h
index 9c50aeedd..d0579a402 100644
--- a/src/s2wasm.h
+++ b/src/s2wasm.h
@@ -1182,6 +1182,10 @@ class S2WasmBuilder {
       } else if (match("end_block")) {
         auto* block = bstack.back()->cast<Block>();
         block->finalize(block->type);
+        if (isConcreteWasmType(block->type) && block->list.size() == 0) {
+          // empty blocks that return a value are not valid, fix that up
+          block->list.push_back(allocator->alloc<Unreachable>());
+        }
         bstack.pop_back();
       } else if (peek(".LBB")) {
         // FIXME legacy tests: it can be leftover from "loop" or "block", but it can be a label too
diff --git a/src/wasm-validator.h b/src/wasm-validator.h
index 00a35fb24..29787a510 100644
--- a/src/wasm-validator.h
+++ b/src/wasm-validator.h
@@ -142,6 +142,9 @@ public:
         }
       }
     }
+    if (isConcreteWasmType(curr->type)) {
+      shouldBeTrue(curr->list.size() > 0, curr, "block with a value must not be empty");
+    }
   }
 
   static void visitPreLoop(WasmValidator* self, Expression** currp) {