diff --git a/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp b/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
--- a/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
+++ b/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
@@ -172,6 +172,9 @@
                                Location loc) -> Optional<Value> {
     if (inputs.size() != 1)
       return llvm::None;
+    // Dialect cast does not support index type.
+    if (resultType == builder.getIndexType())
+      return inputs[0];
     // FIXME: These should check LLVM::DialectCastOp can actually be constructed
     // from the input and result.
     return builder.create<LLVM::DialectCastOp>(loc, resultType, inputs[0])
@@ -182,6 +185,9 @@
                                Location loc) -> Optional<Value> {
     if (inputs.size() != 1)
       return llvm::None;
+    // Dialect cast does not support index type.
+    if (resultType == builder.getIndexType())
+      return inputs[0];
     // FIXME: These should check LLVM::DialectCastOp can actually be constructed
     // from the input and result.
     return builder.create<LLVM::DialectCastOp>(loc, resultType, inputs[0])
diff --git a/mlir/test/Conversion/StandardToLLVM/dialect-cast.mlir b/mlir/test/Conversion/StandardToLLVM/dialect-cast.mlir
new file mode 100644
--- /dev/null
+++ b/mlir/test/Conversion/StandardToLLVM/dialect-cast.mlir
@@ -0,0 +1,13 @@
+// RUN: mlir-opt %s --convert-std-and-scf-to-llvm --split-input-file -verify-diagnostics | FileCheck %s
+
+// CHECK-LABEL: @no_index_cast
+func @no_index_cast() {
+  // CHECK: not mlir.llvm.cast
+  %c0 = constant 0 : index
+  %c1 = constant 1 : index
+  scf.for %i = %c0 to %c1 step %c1 iter_args(%aggr = %c0) {
+    %c123 = constant 123 : index
+    scf.yield %c123 : index
+  }
+}
+