diff --git a/compiler/sizealignoffsetimpl.nim b/compiler/sizealignoffsetimpl.nim
index 4d73923db7939..baa2c40f98d38 100644
--- a/compiler/sizealignoffsetimpl.nim
+++ b/compiler/sizealignoffsetimpl.nim
@@ -164,7 +164,7 @@ proc computeObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode, packed: bool, a
     accum.maxAlign = szUnknownSize
     accum.offset = szUnknownSize
 
-proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; accum: var OffsetAccum) =
+proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; packed: bool; accum: var OffsetAccum) =
   ## ``accum.offset`` will the offset from the larget member of the union.
   case n.kind
   of nkRecCase:
@@ -175,7 +175,7 @@ proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; accum: var
     let accumRoot = accum # copy, because each branch should start af the same offset
     for i, child in n.sons:
       var branchAccum = accumRoot
-      computeUnionObjectOffsetsFoldFunction(conf, child, branchAccum)
+      computeUnionObjectOffsetsFoldFunction(conf, child, packed, branchAccum)
       accum.mergeBranch(branchAccum)
   of nkSym:
     var size = szUnknownSize
@@ -183,7 +183,7 @@ proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; accum: var
     if n.sym.bitsize == 0: # 0 represents bitsize not set
       computeSizeAlign(conf, n.sym.typ)
       size = n.sym.typ.size.int
-      align = n.sym.typ.align.int
+      align = if packed: 1 else: n.sym.typ.align.int
     accum.align(align)
     if n.sym.alignment > 0:
       accum.align(n.sym.alignment)
@@ -365,16 +365,14 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
         else:
           OffsetAccum(maxAlign: 1)
       if tfUnion in typ.flags:
-        if tfPacked in typ.flags:
-          let info = if typ.sym != nil: typ.sym.info else: unknownLineInfo
-          localError(conf, info, "union type may not be packed.")
-          accum = OffsetAccum(offset: szUnknownSize, maxAlign: szUnknownSize)
-        elif accum.offset != 0:
+        if accum.offset != 0:
           let info = if typ.sym != nil: typ.sym.info else: unknownLineInfo
           localError(conf, info, "union type may not have an object header")
           accum = OffsetAccum(offset: szUnknownSize, maxAlign: szUnknownSize)
+        elif tfPacked in typ.flags:
+          computeUnionObjectOffsetsFoldFunction(conf, typ.n, true, accum)
         else:
-          computeUnionObjectOffsetsFoldFunction(conf, typ.n, accum)
+          computeUnionObjectOffsetsFoldFunction(conf, typ.n, false, accum)
       elif tfPacked in typ.flags:
         accum.maxAlign = 1
         computeObjectOffsetsFoldFunction(conf, typ.n, true, accum)
diff --git a/tests/misc/tsizeof.nim b/tests/misc/tsizeof.nim
index 579532fcca56e..c1d9df6117eea 100644
--- a/tests/misc/tsizeof.nim
+++ b/tests/misc/tsizeof.nim
@@ -694,3 +694,13 @@ type
 
 doAssert sizeof(O0) == 1
 doAssert sizeof(T0) == 1
+
+
+type
+  # this thing may not have padding bytes at the end
+  PackedUnion* {.union, packed.} = object
+    a*: array[11, byte]
+    b*: int64
+
+doAssert sizeof(PackedUnion) == 11
+doAssert alignof(PackedUnion) == 1