diff --git a/src/librustc_trans/abi.rs b/src/librustc_trans/abi.rs
index c2b040c32f6a3..42289ec094f32 100644
--- a/src/librustc_trans/abi.rs
+++ b/src/librustc_trans/abi.rs
@@ -35,6 +35,7 @@ use std::cmp;
 
 pub use syntax::abi::Abi;
 pub use rustc::ty::layout::{FAT_PTR_ADDR, FAT_PTR_EXTRA};
+use rustc::ty::layout::Layout;
 
 #[derive(Clone, Copy, PartialEq, Debug)]
 enum ArgKind {
@@ -317,6 +318,11 @@ impl FnType {
                 if ty.is_integral() {
                     arg.signedness = Some(ty.is_signed());
                 }
+                // Rust enum types that map onto C enums also need to follow
+                // the target ABI zero-/sign-extension rules.
+                if let Layout::CEnum { signed, .. } = *ccx.layout_of(ty) {
+                    arg.signedness = Some(signed);
+                }
                 if llsize_of_real(ccx, arg.ty) == 0 {
                     // For some forsaken reason, x86_64-pc-windows-gnu
                     // doesn't ignore zero-sized struct arguments.