diff --git a/lib/Optimizer/CodeGen/CodeGen.cpp b/lib/Optimizer/CodeGen/CodeGen.cpp
index 73c6f14..1e1f3ee 100644
--- a/lib/Optimizer/CodeGen/CodeGen.cpp
+++ b/lib/Optimizer/CodeGen/CodeGen.cpp
@@ -827,6 +827,20 @@
   }
 };
 
+/// Lower fir.len_param_index
+struct LenParamIndexOpConversion
+    : public FIROpConversion<fir::LenParamIndexOp> {
+  using FIROpConversion::FIROpConversion;
+
+  // FIXME: this should be specialized by the runtime target
+  mlir::LogicalResult
+  matchAndRewrite(fir::LenParamIndexOp lenp, OpAdaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    return rewriter.notifyMatchFailure(
+        lenp, "fir.len_param_index codegen is not implemented yet");
+  }
+};
+
 /// Lower `fir.gentypedesc` to a global constant.
 struct GenTypeDescOpConversion : public FIROpConversion<fir::GenTypeDescOp> {
   using FIROpConversion::FIROpConversion;
@@ -2179,14 +2193,15 @@
         EmboxProcOpConversion, ExtractValueOpConversion, FieldIndexOpConversion,
         FirEndOpConversion, HasValueOpConversion, GenTypeDescOpConversion,
         GlobalLenOpConversion, GlobalOpConversion, InsertOnRangeOpConversion,
-        InsertValueOpConversion, IsPresentOpConversion, LoadOpConversion,
-        NegcOpConversion, NoReassocOpConversion, MulcOpConversion,
-        SelectCaseOpConversion, SelectOpConversion, SelectRankOpConversion,
-        SelectTypeOpConversion, ShapeOpConversion, ShapeShiftOpConversion,
-        ShiftOpConversion, SliceOpConversion, StoreOpConversion,
-        StringLitOpConversion, SubcOpConversion, UnboxCharOpConversion,
-        UnboxProcOpConversion, UndefOpConversion, UnreachableOpConversion,
-        ZeroOpConversion>(typeConverter);
+        InsertValueOpConversion, IsPresentOpConversion,
+        LenParamIndexOpConversion, LoadOpConversion, NegcOpConversion,
+        NoReassocOpConversion, MulcOpConversion, SelectCaseOpConversion,
+        SelectOpConversion, SelectRankOpConversion, SelectTypeOpConversion,
+        ShapeOpConversion, ShapeShiftOpConversion, ShiftOpConversion,
+        SliceOpConversion, StoreOpConversion, StringLitOpConversion,
+        SubcOpConversion, UnboxCharOpConversion, UnboxProcOpConversion,
+        UndefOpConversion, UnreachableOpConversion, ZeroOpConversion>(
+        typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
diff --git a/test/Fir/convert-to-llvm-invalid.fir b/test/Fir/convert-to-llvm-invalid.fir
index c916c58..248ca5e 100644
--- a/test/Fir/convert-to-llvm-invalid.fir
+++ b/test/Fir/convert-to-llvm-invalid.fir
@@ -86,6 +86,16 @@
 
 // -----
 
+// Test `fir.len_param_index` conversion failure. Not implemented yet.
+
+func @lenparamindex() {
+  // expected-error@+1{{failed to legalize operation 'fir.len_param_index'}}
+  %0 = fir.len_param_index l1, !fir.type<twolens(l1:i32, l2:i32){i:i32, f:f32, l:i64}>
+  return
+}
+
+// -----
+
 // Test `fir.gentypedesc` conversion failure. Not implemented yet.
 
 func @gentypedesc() {