diff --git a/flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp b/flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp
index 0c34c8221aeda6..ace63a970db932 100644
--- a/flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp
+++ b/flang/lib/Optimizer/HLFIR/Transforms/SimplifyHLFIRIntrinsics.cpp
@@ -108,7 +108,6 @@ class SumAsElementalConversion : public mlir::OpRewritePattern<hlfir::SumOp> {
     mlir::Value mask = sum.getMask();
     mlir::Value dim = sum.getDim();
     int64_t dimVal = fir::getIntIfConstant(dim).value_or(0);
-    assert(dimVal > 0 && "DIM must be present and a positive constant");
     mlir::Value resultShape, dimExtent;
     std::tie(resultShape, dimExtent) =
         genResultShape(loc, builder, array, dimVal);
@@ -235,6 +234,9 @@ class SumAsElementalConversion : public mlir::OpRewritePattern<hlfir::SumOp> {
     mlir::Value inShape = hlfir::genShape(loc, builder, array);
     llvm::SmallVector<mlir::Value> inExtents =
         hlfir::getExplicitExtentsFromShape(inShape, builder);
+    assert(dimVal > 0 && dimVal <= static_cast<int64_t>(inExtents.size()) &&
+           "DIM must be present and a positive constant not exceeding "
+           "the array's rank");
     if (inShape.getUses().empty())
       inShape.getDefiningOp()->erase();
 
@@ -348,12 +350,20 @@ class SimplifyHLFIRIntrinsics
     // would avoid creating a temporary for the elemental array expression.
     target.addDynamicallyLegalOp<hlfir::SumOp>([](hlfir::SumOp sum) {
       if (mlir::Value dim = sum.getDim()) {
-        if (fir::getIntIfConstant(dim)) {
+        if (auto dimVal = fir::getIntIfConstant(dim)) {
           if (!fir::isa_trivial(sum.getType())) {
             // Ignore the case SUM(a, DIM=X), where 'a' is a 1D array.
             // It is only legal when X is 1, and it should probably be
             // canonicalized into SUM(a).
-            return false;
+            fir::SequenceType arrayTy = mlir::cast<fir::SequenceType>(
+                hlfir::getFortranElementOrSequenceType(
+                    sum.getArray().getType()));
+            if (*dimVal > 0 && *dimVal <= arrayTy.getDimension()) {
+              // Ignore SUMs with illegal DIM values.
+              // They may appear in dead code,
+              // and they do not have to be converted.
+              return false;
+            }
           }
         }
       }
diff --git a/flang/test/HLFIR/simplify-hlfir-intrinsics-sum.fir b/flang/test/HLFIR/simplify-hlfir-intrinsics-sum.fir
index 703b6673154f3f..313e54d5d0c4af 100644
--- a/flang/test/HLFIR/simplify-hlfir-intrinsics-sum.fir
+++ b/flang/test/HLFIR/simplify-hlfir-intrinsics-sum.fir
@@ -411,3 +411,21 @@ func.func @sum_non_const_dim(%arg0: !fir.box<!fir.array<3xi32>>, %dim: i32) {
 // CHECK:           %[[VAL_2:.*]] = hlfir.sum %[[VAL_0]] dim %[[VAL_1]] : (!fir.box<!fir.array<3xi32>>, i32) -> i32
 // CHECK:           return
 // CHECK:         }
+
+// negative: invalid dim==0
+func.func @sum_invalid_dim0(%arg0: !hlfir.expr<2x3xi32>) {
+  %cst = arith.constant 0 : i32
+  %res = hlfir.sum %arg0 dim %cst : (!hlfir.expr<2x3xi32>, i32) -> !hlfir.expr<3xi32>
+  return
+}
+// CHECK-LABEL:   func.func @sum_invalid_dim0(
+// CHECK:           hlfir.sum %{{.*}} dim %{{.*}} : (!hlfir.expr<2x3xi32>, i32) -> !hlfir.expr<3xi32>
+
+// negative: invalid dim>rank
+func.func @sum_invalid_dim_big(%arg0: !hlfir.expr<2x3xi32>) {
+  %cst = arith.constant 3 : i32
+  %res = hlfir.sum %arg0 dim %cst : (!hlfir.expr<2x3xi32>, i32) -> !hlfir.expr<3xi32>
+  return
+}
+// CHECK-LABEL:   func.func @sum_invalid_dim_big(
+// CHECK:           hlfir.sum %{{.*}} dim %{{.*}} : (!hlfir.expr<2x3xi32>, i32) -> !hlfir.expr<3xi32>