diff --git a/mlir/lib/IR/Operation.cpp b/mlir/lib/IR/Operation.cpp
--- a/mlir/lib/IR/Operation.cpp
+++ b/mlir/lib/IR/Operation.cpp
@@ -410,7 +410,7 @@
 /// This is a trait method invoked when an operation is added to a block.  We
 /// keep the block pointer up to date.
 void llvm::ilist_traits<::mlir::Operation>::addNodeToList(Operation *op) {
-  assert(!op->getBlock() && "already in an operation block!");
+  assert(!op->getBlock() && "operation already exists in some block!");
   op->block = getContainingBlock();
 
   // Invalidate the order on the operation.