diff --git a/src/Compiler/CodeGen/IlxGen.fs b/src/Compiler/CodeGen/IlxGen.fs
index 939e3da324..0f180c41c9 100644
--- a/src/Compiler/CodeGen/IlxGen.fs
+++ b/src/Compiler/CodeGen/IlxGen.fs
@@ -12031,15 +12031,9 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                         }
 
                     let layout =
-                        // Structs with no instance fields get size 1, pack 0
+                        // Struct unions always carry a hidden tag field, so never emit size 1 here
                         if isStructTy g thisTy then
-                            if
-                                (tycon.AllFieldsArray.Length = 0
-                                 || tycon.AllFieldsArray |> Array.exists (fun f -> not f.IsStatic))
-                                && (alternatives
-                                    |> Array.collect (fun a -> a.FieldDefs)
-                                    |> Array.exists (fun fd -> not fd.ILField.IsStatic))
-                            then
+                            if tycon.IsUnionTycon then
                                 ILTypeDefLayout.Sequential { Size = None; Pack = None }
                             else
                                 ILTypeDefLayout.Sequential { Size = Some 1; Pack = Some 0us }
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs
index 9314881045..16448e181d 100644
--- a/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs
@@ -397,7 +397,7 @@ module CustomAttributes_Basic =
         ]
 
     [<Fact>]
-    let ``StructLayoutAttribute has size=1 for struct DUs with no instance fields`` () =
+    let ``StructLayoutAttribute doesn't have size=1 for struct DUs with no instance fields`` () =
         Fsx """
         [<Struct>] type Option<'T> = None | Some
         """
@@ -413,8 +413,6 @@ module CustomAttributes_Basic =
                    [runtime]System.IComparable,
                    [runtime]System.Collections.IStructuralComparable
   {
-    .pack 0
-    .size 1
     .custom instance void [FSharp.Core]Microsoft.FSharp.Core.StructAttribute::.ctor() = ( 01 00 00 00 ) 
     .custom instance void [runtime]System.Diagnostics.DebuggerDisplayAttribute::.ctor(string) = ( 01 00 15 7B 5F 5F 44 65 62 75 67 44 69 73 70 6C   
                                                                                                   61 79 28 29 2C 6E 71 7D 00 00 )                   
@@ -426,4 +424,4 @@ module CustomAttributes_Basic =
       .field public static literal int32 Some = int32(0x00000001)
     } 
         """
-        ]
\ No newline at end of file
+        ]