diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index 65d4d5120d0..9e834b83df4 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -351,6 +351,11 @@ fn llvm_type(&self, cx: &CodegenCx<'ll, 'tcx>) -> &'ll Type {
                     // guarnateeing that we generate ABI-compatible LLVM IR. Things get tricky for
                     // aggregates...
                     if matches!(arg.layout.abi, abi::Abi::Aggregate { .. }) {
+                        assert!(
+                            arg.layout.is_sized(),
+                            "`PassMode::Direct` for unsized type: {}",
+                            arg.layout.ty
+                        );
                         // This really shouldn't happen, since `immediate_llvm_type` will use
                         // `layout.fields` to turn this Rust type into an LLVM type. This means all
                         // sorts of Rust type details leak into the ABI. However wasm sadly *does*