diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 7ce110dcbfc4..40ae0a13c729 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -502,6 +502,7 @@ pub fn codegen_rvalue_operand(
             }
 
             mir::Rvalue::NullaryOp(mir::NullOp::SizeOf, ty) => {
+                let ty = self.monomorphize(&ty);
                 assert!(bx.cx().type_is_sized(ty));
                 let val = bx.cx().const_usize(bx.cx().layout_of(ty).size.bytes());
                 let tcx = self.cx.tcx();