diff --git a/src/libcore/ffi.rs b/src/libcore/ffi.rs
index eda0e7c518c58..2ebc8794ff915 100644
--- a/src/libcore/ffi.rs
+++ b/src/libcore/ffi.rs
@@ -290,16 +290,20 @@ impl_va_arg_safe!{i8, i16, i32, i64, usize}
 impl_va_arg_safe!{u8, u16, u32, u64, isize}
 impl_va_arg_safe!{f64}
 
+// FIXME(ahomescu): this also enables VaArgSafe for fat pointers,
+// which we'd like to avoid. There is no way to currently implement
+// a trait exclusively for thin pointers, but RFC #2580 changes this
+// so we can fix our implementation once 2580 is implemented.
 #[unstable(feature = "c_variadic",
            reason = "the `c_variadic` feature has not been properly tested on \
                      all supported platforms",
            issue = "44930")]
-impl<T> sealed_trait::VaArgSafe for *mut T {}
+impl<T: ?Sized> sealed_trait::VaArgSafe for *mut T {}
 #[unstable(feature = "c_variadic",
            reason = "the `c_variadic` feature has not been properly tested on \
                      all supported platforms",
            issue = "44930")]
-impl<T> sealed_trait::VaArgSafe for *const T {}
+impl<T: ?Sized> sealed_trait::VaArgSafe for *const T {}
 
 #[unstable(feature = "c_variadic",
            reason = "the `c_variadic` feature has not been properly tested on \
diff --git a/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/checkrust.rs b/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/checkrust.rs
index a0a5b141ec0e1..f57e513d7cad5 100644
--- a/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/checkrust.rs
+++ b/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/checkrust.rs
@@ -1,6 +1,7 @@
 #![crate_type = "staticlib"]
 #![feature(c_variadic)]
 #![feature(rustc_private)]
+#![feature(extern_types)]
 
 extern crate libc;
 
@@ -8,6 +9,11 @@ use libc::{c_char, c_double, c_int, c_long, c_longlong};
 use std::ffi::VaList;
 use std::ffi::{CString, CStr};
 
+extern {
+    // This is an unsized type, used to test pointer-to-unsized-types
+    type Opaque;
+}
+
 macro_rules! continue_if {
     ($cond:expr) => {
         if !($cond) {
@@ -91,3 +97,9 @@ pub unsafe extern "C" fn check_varargs_1(_: c_int, mut ap: ...) -> usize {
 pub unsafe extern "C" fn check_varargs_2(_: c_int, _ap: ...) -> usize {
     0
 }
+
+#[no_mangle]
+pub unsafe extern "C" fn check_varargs_3(_: c_int, mut ap: ...) -> usize {
+    continue_if!(ap.arg::<*const Opaque>() as usize == 0x1234usize);
+    0
+}
diff --git a/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/test.c b/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/test.c
index 91b060dce26f4..681c9dc7913d3 100644
--- a/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/test.c
+++ b/src/test/run-make-fulldeps/c-link-to-rust-va-list-fn/test.c
@@ -11,6 +11,9 @@ extern size_t check_list_copy_0(va_list ap);
 extern size_t check_varargs_0(int fixed, ...);
 extern size_t check_varargs_1(int fixed, ...);
 extern size_t check_varargs_2(int fixed, ...);
+extern size_t check_varargs_3(int fixed, ...);
+
+struct opaque_t;
 
 int test_rust(size_t (*fn)(va_list), ...) {
     size_t ret = 0;
@@ -36,5 +39,9 @@ int main(int argc, char* argv[]) {
 
     assert(check_varargs_2(0, "All", "of", "these", "are", "ignored", ".") == 0);
 
+    // Test pointer-to-unsized-type values
+    struct opaque_t *op = (struct opaque_t*) 0x1234;
+    assert(check_varargs_3(0, op) == 0);
+
     return 0;
 }