diff --git a/mlir/lib/Reducer/ReductionTreePass.cpp b/mlir/lib/Reducer/ReductionTreePass.cpp
--- a/mlir/lib/Reducer/ReductionTreePass.cpp
+++ b/mlir/lib/Reducer/ReductionTreePass.cpp
@@ -217,7 +217,11 @@
   Operation *topOperation = getOperation();
   while (topOperation->getParentOp() != nullptr)
     topOperation = topOperation->getParentOp();
-  ModuleOp module = cast<ModuleOp>(topOperation);
+  ModuleOp module = dyn_cast<ModuleOp>(topOperation);
+  if (!module)
+    return emitError(getOperation()->getLoc())
+               << "top-level op must be 'builtin.module'",
+           signalPassFailure();
 
   SmallVector<Operation *, 8> workList;
   workList.push_back(getOperation());
diff --git a/mlir/lib/Tools/mlir-reduce/MlirReduceMain.cpp b/mlir/lib/Tools/mlir-reduce/MlirReduceMain.cpp
--- a/mlir/lib/Tools/mlir-reduce/MlirReduceMain.cpp
+++ b/mlir/lib/Tools/mlir-reduce/MlirReduceMain.cpp
@@ -23,16 +23,27 @@
 #include "mlir/Support/FileUtilities.h"
 #include "mlir/Support/LogicalResult.h"
 #include "llvm/Support/InitLLVM.h"
+#include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/ToolOutputFile.h"
 
 using namespace mlir;
 
 // Parse and verify the input MLIR file.
 static LogicalResult loadModule(MLIRContext &context,
-                                OwningOpRef<ModuleOp> &module,
+                                OwningOpRef<Operation *> &op,
                                 StringRef inputFilename) {
-  module = parseSourceFile<ModuleOp>(inputFilename, &context);
-  if (!module)
+  // Set up the input file.
+  std::string errorMessage;
+  auto file = openInputFile(inputFilename, &errorMessage);
+  if (!file) {
+    llvm::errs() << errorMessage << "\n";
+    return failure();
+  }
+
+  llvm::SourceMgr sourceMgr;
+  sourceMgr.AddNewSourceBuffer(std::move(file), SMLoc());
+  op = parseSourceFileForTool(sourceMgr, &context);
+  if (!op)
     return failure();
 
   return success();
@@ -62,6 +73,7 @@
   registerReducerPasses();
 
   PassPipelineCLParser parser("", "Reduction Passes to Run");
+  registerToolParserCLOptions();
   llvm::cl::ParseCommandLineOptions(argc, argv,
                                     "MLIR test case reduction tool.\n");
 
@@ -76,8 +88,8 @@
   if (!output)
     return failure();
 
-  OwningOpRef<ModuleOp> moduleRef;
-  if (failed(loadModule(context, moduleRef, inputFilename)))
+  OwningOpRef<Operation *> opRef;
+  if (failed(loadModule(context, opRef, inputFilename)))
     return failure();
 
   auto errorHandler = [&](const Twine &msg) {
@@ -85,16 +97,16 @@
   };
 
   // Reduction pass pipeline.
-  PassManager pm(&context);
+  PassManager pm(&context, opRef.get()->getName().getStringRef());
   if (failed(parser.addToPipeline(pm, errorHandler)))
     return failure();
 
-  OwningOpRef<ModuleOp> m = moduleRef.get().clone();
+  OwningOpRef<Operation *> op = opRef.get()->clone();
 
-  if (failed(pm.run(m.get())))
+  if (failed(pm.run(op.get())))
     return failure();
 
-  m->print(output->os());
+  op.get()->print(output->os());
   output->keep();
 
   return success();
diff --git a/mlir/test/mlir-reduce/invalid.mlir b/mlir/test/mlir-reduce/invalid.mlir
new file mode 100644
--- /dev/null
+++ b/mlir/test/mlir-reduce/invalid.mlir
@@ -0,0 +1,8 @@
+// UNSUPPORTED: system-windows
+// RUN: not mlir-reduce -opt-reduction-pass --no-implicit-module %s |& FileCheck %s --check-prefix=CHECK-PASS
+// RUN: not mlir-reduce -reduction-tree --no-implicit-module %s |& FileCheck %s --check-prefix=CHECK-TREE
+
+// The reduction passes are currently restricted to 'builtin.module'.
+// CHECK-PASS: error: Can't add pass '{{.+}}' restricted to 'builtin.module' on a PassManager intended to run on 'func.func'
+// CHECK-TREE: error: top-level op must be 'builtin.module'
+func.func private @foo()