diff --git a/base/compiler/tfuncs.jl b/base/compiler/tfuncs.jl
index ac8aa0c639dc5..3b5f53a27c516 100644
--- a/base/compiler/tfuncs.jl
+++ b/base/compiler/tfuncs.jl
@@ -380,7 +380,7 @@ function sizeof_tfunc(@nospecialize(x),)
         x = unwrap_unionall(t)
         if exact && isa(x, Union)
             isinline, sz, _ = uniontype_layout(x)
-            return isinline ? Const(Int(sz)) : Bottom
+            return isinline ? Const(Int(Core.sizeof(x))) : Bottom
         end
         isa(x, DataType) || return Int
         (isconcretetype(x) || isprimitivetype(x)) && return _const_sizeof(x)
diff --git a/src/builtins.c b/src/builtins.c
index 6d5f3f2779a12..8cbce6c5b6188 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -399,8 +399,8 @@ JL_CALLABLE(jl_f_sizeof)
     jl_value_t *x = args[0];
     if (jl_is_unionall(x) || jl_is_uniontype(x)) {
         x = jl_unwrap_unionall(x);
-        size_t elsize = 0, al = 0;
-        int isinline = jl_islayout_inline(x, &elsize, &al);
+        size_t elsize = 0;
+        int isinline = jl_uniontype_size(x, &elsize);
         if (isinline)
             return jl_box_long(elsize);
         if (!jl_is_datatype(x))
diff --git a/src/datatype.c b/src/datatype.c
index eeceae946b74e..3428e7479e9ed 100644
--- a/src/datatype.c
+++ b/src/datatype.c
@@ -235,13 +235,13 @@ STATIC_INLINE void jl_maybe_allocate_singleton_instance(jl_datatype_t *st)
     }
 }
 
-static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbytes, size_t *align) JL_NOTSAFEPOINT
+static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbytes, size_t *align, int asfield) JL_NOTSAFEPOINT
 {
     if (jl_is_uniontype(ty)) {
-        unsigned na = union_isinlinable(((jl_uniontype_t*)ty)->a, 1, nbytes, align);
+        unsigned na = union_isinlinable(((jl_uniontype_t*)ty)->a, 1, nbytes, align, asfield);
         if (na == 0)
             return 0;
-        unsigned nb = union_isinlinable(((jl_uniontype_t*)ty)->b, 1, nbytes, align);
+        unsigned nb = union_isinlinable(((jl_uniontype_t*)ty)->b, 1, nbytes, align, asfield);
         if (nb == 0)
             return 0;
         return na + nb;
@@ -249,6 +249,9 @@ static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbyte
     if (jl_is_datatype(ty) && jl_datatype_isinlinealloc(ty) && (!pointerfree || ((jl_datatype_t*)ty)->layout->npointers == 0)) {
         size_t sz = jl_datatype_size(ty);
         size_t al = jl_datatype_align(ty);
+        // primitive types in struct slots need their sizes aligned. issue #37974
+        if (asfield && jl_is_primitivetype(ty))
+            sz = LLT_ALIGN(sz, al);
         if (*nbytes < sz)
             *nbytes = sz;
         if (*align < al)
@@ -258,9 +261,15 @@ static unsigned union_isinlinable(jl_value_t *ty, int pointerfree, size_t *nbyte
     return 0;
 }
 
+int jl_uniontype_size(jl_value_t *ty, size_t *sz) JL_NOTSAFEPOINT
+{
+    size_t al = 0;
+    return union_isinlinable(ty, 0, sz, &al, 0) != 0;
+}
+
 JL_DLLEXPORT int jl_islayout_inline(jl_value_t *eltype, size_t *fsz, size_t *al) JL_NOTSAFEPOINT
 {
-    unsigned countbits = union_isinlinable(eltype, 0, fsz, al);
+    unsigned countbits = union_isinlinable(eltype, 0, fsz, al, 1);
     return (countbits > 0 && countbits < 127) ? countbits : 0;
 }
 
diff --git a/src/julia.h b/src/julia.h
index 9b55c5db0cf98..2cd47e80f6a37 100644
--- a/src/julia.h
+++ b/src/julia.h
@@ -1398,6 +1398,7 @@ JL_DLLEXPORT void        jl_set_nth_field(jl_value_t *v, size_t i,
 JL_DLLEXPORT int         jl_field_isdefined(jl_value_t *v, size_t i) JL_NOTSAFEPOINT;
 JL_DLLEXPORT jl_value_t *jl_get_field(jl_value_t *o, const char *fld);
 JL_DLLEXPORT jl_value_t *jl_value_ptr(jl_value_t *a);
+int jl_uniontype_size(jl_value_t *ty, size_t *sz) JL_NOTSAFEPOINT;
 JL_DLLEXPORT int jl_islayout_inline(jl_value_t *eltype, size_t *fsz, size_t *al) JL_NOTSAFEPOINT;
 
 // arrays
diff --git a/test/compiler/codegen.jl b/test/compiler/codegen.jl
index 2ccffc40b2f16..9b147fd0b571b 100644
--- a/test/compiler/codegen.jl
+++ b/test/compiler/codegen.jl
@@ -500,3 +500,15 @@ end
 let f(@nospecialize(x)) = x===Base.ImmutableDict(Int128=>:big)
     @test !f(Dict(Int=>Int))
 end
+
+# issue #37974
+primitive type UInt24 24 end
+let a = Core.Intrinsics.trunc_int(UInt24, 3),
+    f(t) = t[2]
+    @test f((a, true)) === true
+    @test f((a, false)) === false
+    @test sizeof(Tuple{UInt24,Bool}) == 8
+    @test sizeof(UInt24) == 3
+    @test sizeof(Union{UInt8,UInt24}) == 3
+    @test sizeof(Base.RefValue{Union{UInt8,UInt24}}) == 8
+end