diff --git a/src/llvm_backend_expr.cpp b/src/llvm_backend_expr.cpp
index f843dfa92..962082cae 100644
--- a/src/llvm_backend_expr.cpp
+++ b/src/llvm_backend_expr.cpp
@@ -2469,6 +2469,17 @@ gb_internal lbValue lb_emit_conv(lbProcedure *p, lbValue value, Type *t) {
 	return {};
 }
 
+gb_internal lbValue lb_emit_c_vararg(lbProcedure *p, lbValue arg, Type *type) {
+	Type *core = core_type(type);
+	if (core->kind == Type_BitSet) {
+		core = core_type(bit_set_to_int(core));
+		arg  = lb_emit_transmute(p, arg, core);
+	}
+
+	Type *promoted = c_vararg_promote_type(core);
+	return lb_emit_conv(p, arg, promoted);
+}
+
 gb_internal lbValue lb_compare_records(lbProcedure *p, TokenKind op_kind, lbValue left, lbValue right, Type *type) {
 	GB_ASSERT((is_type_struct(type) || is_type_union(type)) && is_type_comparable(type));
 	lbValue left_ptr  = lb_address_from_load_or_generate_local(p, left);
diff --git a/src/llvm_backend_proc.cpp b/src/llvm_backend_proc.cpp
index ee3ed1995..3326b4041 100644
--- a/src/llvm_backend_proc.cpp
+++ b/src/llvm_backend_proc.cpp
@@ -3569,9 +3569,9 @@ gb_internal lbValue lb_build_call_expr_internal(lbProcedure *p, Ast *expr) {
 							if (is_type_untyped_nil(arg.type)) {
 								arg = lb_const_nil(p->module, t_rawptr);
 							}
-							array_add(&args, lb_emit_conv(p, arg, c_vararg_promote_type(default_type(arg.type))));
+							array_add(&args, lb_emit_c_vararg(p, arg, arg.type));
 						} else {
-							array_add(&args, lb_emit_conv(p, arg, c_vararg_promote_type(elem_type)));
+							array_add(&args, lb_emit_c_vararg(p, arg, elem_type));
 						}
 					}
 					break;
@@ -3697,15 +3697,15 @@ gb_internal lbValue lb_build_call_expr_internal(lbProcedure *p, Ast *expr) {
 						if (is_type_untyped_nil(arg.type)) {
 							arg = lb_const_nil(p->module, t_rawptr);
 						}
-						array_add(&args, lb_emit_conv(p, arg, c_vararg_promote_type(default_type(arg.type))));
+						array_add(&args, lb_emit_c_vararg(p, arg, arg.type));
 					} else {
-						array_add(&args, lb_emit_conv(p, arg, c_vararg_promote_type(elem_type)));
+						array_add(&args, lb_emit_c_vararg(p, arg, elem_type));
 					}
 				}
 			} else {
 				lbValue value = lb_build_expr(p, fv->value);
 				GB_ASSERT(!is_type_tuple(value.type));
-				array_add(&args, lb_emit_conv(p, value, c_vararg_promote_type(value.type)));
+				array_add(&args, lb_emit_c_vararg(p, value, value.type));
 			}
 		} else {
 			lbValue value = lb_build_expr(p, fv->value);
diff --git a/src/types.cpp b/src/types.cpp
index da5099d22..21dd6ad39 100644
--- a/src/types.cpp
+++ b/src/types.cpp
@@ -2960,10 +2960,7 @@ gb_internal Type *c_vararg_promote_type(Type *type) {
 	GB_ASSERT(type != nullptr);
 
 	Type *core = core_type(type);
-
-	if (core->kind == Type_BitSet) {
-		core = core_type(bit_set_to_int(core));
-	}
+	GB_ASSERT(core->kind != Type_BitSet);
 
 	if (core->kind == Type_Basic) {
 		switch (core->Basic.kind) {