diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index 9c54b86dcd3..fe2cba1180c 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -22,6 +22,7 @@ import middle::metadata;
 import util::common::*;
 import util::data::interner;
 import pretty::ppaux::ty_to_str;
+import pretty::ppaux::mode_str_1;
 
 
 export node_id_to_monotype;
@@ -292,6 +293,7 @@ tag type_err {
     terr_meth_count;
     terr_obj_meths(ast::ident, ast::ident);
     terr_arg_count;
+    terr_mode_mismatch(mode, mode);
 }
 
 type ty_param_count_and_ty = tup(uint, t);
@@ -1984,9 +1986,8 @@ mod unify {
 
             auto result_mode;
             if (expected_input.mode != actual_input.mode) {
-                // FIXME this is the wrong error
-
-                ret fn_common_res_err(ures_err(terr_arg_count));
+                ret fn_common_res_err(ures_err(terr_mode_mismatch(
+                                    expected_input.mode, actual_input.mode)));
             } else { result_mode = expected_input.mode; }
             auto result = unify_step(cx, expected_input.ty, actual_input.ty);
             alt (result) {
@@ -2584,6 +2585,11 @@ fn type_err_to_str(&ty::type_err err) -> str {
             ret "expected an obj with method '" + e_meth +
                     "' but found one with method '" + a_meth + "'";
         }
+        case (terr_mode_mismatch(?e_mode, ?a_mode)) {
+            ret "expected argument mode " + mode_str_1(e_mode) + " but found "
+                + mode_str_1(a_mode);
+            fail;
+        }
     }
 }
 
diff --git a/src/comp/pretty/ppaux.rs b/src/comp/pretty/ppaux.rs
index de782e2fe6c..97c932cdd5f 100644
--- a/src/comp/pretty/ppaux.rs
+++ b/src/comp/pretty/ppaux.rs
@@ -5,6 +5,7 @@ import std::str;
 import std::option;
 import std::option::none;
 import std::option::some;
+import middle::ty;
 import middle::ty::*;
 import front::lexer;
 import front::ast;
@@ -18,15 +19,25 @@ import util::common::istr;
 import util::common::uistr;
 import util::common::ty_mach_to_str;
 
+fn mode_str(&ty::mode m) -> str {
+    alt (m) {
+        case (mo_val) { "" }
+        case (mo_alias(false)) { "&" }
+        case (mo_alias(true)) { "&mutable " }
+    }
+}
+
+fn mode_str_1(&ty::mode m) -> str {
+    alt (m) {
+        case (mo_val) { "val" }
+        case (_)      { mode_str(m) }
+    }
+}
+
 fn ty_to_str(&ctxt cx, &t typ) -> str {
     fn fn_input_to_str(&ctxt cx, &rec(middle::ty::mode mode, t ty) input) ->
        str {
-        auto s =
-            alt (input.mode) {
-                case (mo_val) { "" }
-                case (mo_alias(false)) { "&" }
-                case (mo_alias(true)) { "&mutable " }
-            };
+        auto s = mode_str(input.mode);
         ret s + ty_to_str(cx, input.ty);
     }
     fn fn_to_str(&ctxt cx, ast::proto proto, option::t[ast::ident] ident,
diff --git a/src/test/compile-fail/alias-mismatch.rs b/src/test/compile-fail/alias-mismatch.rs
new file mode 100644
index 00000000000..b280351db2b
--- /dev/null
+++ b/src/test/compile-fail/alias-mismatch.rs
@@ -0,0 +1,9 @@
+// error-pattern:expected argument mode
+use std;
+import std::vec::map;
+
+fn main() {
+  fn f(uint i) -> bool { true }
+
+  auto a = map(f, [5u]);
+}
\ No newline at end of file