diff -r 669f47397249 -r 276ad4354069 src/Pure/defs.ML
--- a/src/Pure/defs.ML	Sun Dec 20 13:03:41 2015 +0100
+++ b/src/Pure/defs.ML	Sun Dec 20 13:06:26 2015 +0100
@@ -147,7 +147,8 @@
 fun disjoint_specs context c (i, {description = a, pos = pos_a, lhs = Ts, ...}: spec) =
   Inttab.forall (fn (j, {description = b, pos = pos_b, lhs = Us, ...}: spec) =>
     i = j orelse disjoint_args (Ts, Us) orelse
-      error ("Clash of specifications for " ^ Pretty.str_of (pretty_item context c) ^ ":\n" ^
+      error ("Clash of specifications for " ^
+        Pretty.unformatted_string_of (pretty_item context c) ^ ":\n" ^
         "  " ^ quote a ^ Position.here pos_a ^ "\n" ^
         "  " ^ quote b ^ Position.here pos_b));