diff --git a/mlir/test/lit.cfg.py b/mlir/test/lit.cfg.py
--- a/mlir/test/lit.cfg.py
+++ b/mlir/test/lit.cfg.py
@@ -84,7 +84,7 @@
 # FileCheck -enable-var-scope is enabled by default in MLIR test
 # This option avoids to accidentally reuse variable across -LABEL match,
 # it can be explicitly opted-in by prefixing the variable name with $
-config.environment['FILECHECK_OPTS'] = "-enable-var-scope"
+config.environment['FILECHECK_OPTS'] = "-enable-var-scope --allow-unused-prefixes=false"
 
 
 # LLVM can be configured with an empty default triple