diff --git a/src/comp/front/extfmt.rs b/src/comp/front/extfmt.rs
index 58949518649c..64fe7b6df5d3 100644
--- a/src/comp/front/extfmt.rs
+++ b/src/comp/front/extfmt.rs
@@ -74,11 +74,12 @@ fn expr_to_str(@ast::expr expr) -> str {
                 case (ast::lit_str(?s)) {
                     ret s;
                 }
+                case (_) { /* fallthrough */ }
             }
         }
+        case (_) { /* fallthrough */ }
     }
-    // FIXME: Handle error correctly.
-    log_err "malformed #fmt call";
+    log_err "first argument to #fmt must be a string literal";
     fail;
 }
 
diff --git a/src/test/compile-fail/extfmt-non-literal.rs b/src/test/compile-fail/extfmt-non-literal.rs
new file mode 100644
index 000000000000..4b85498c1ab0
--- /dev/null
+++ b/src/test/compile-fail/extfmt-non-literal.rs
@@ -0,0 +1,9 @@
+// xfail-stage0
+// error-pattern: literal
+
+fn main() {
+  // #fmt's first argument must be a literal.  Hopefully this
+  // restriction can be eased eventually to just require a
+  // compile-time constant.
+  auto x = #fmt("a" + "b");
+}