diff --git a/compiler/pred_table.m b/compiler/pred_table.m
index b9d35d4..4ddf788 100644
--- a/compiler/pred_table.m
+++ b/compiler/pred_table.m
@@ -1224,9 +1224,9 @@ lookup_builtin_pred_proc_id(Module, ModuleName, ProcName, PredOrFunc,
     ->
         PredId = PredIdPrime
     ;
-        string.int_to_string(Arity, ArityStr),
         unexpected($module, $pred,
-            "can't locate " ++ ProcName ++ "/" ++ ArityStr)
+            string.format("can't locate %s.%s/%d",
+                [s(ModuleName), s(ProcName), i(Arity)]))
     ),
     module_info_pred_info(Module, PredId, PredInfo),
     ProcIds = pred_info_procids(PredInfo),
@@ -1236,8 +1236,8 @@ lookup_builtin_pred_proc_id(Module, ModuleName, ProcName, PredOrFunc,
             ProcId = ProcId0
         ;
             unexpected($module, $pred,
-                string.format("expected single mode for %s/%d",
-                    [s(ProcName), i(Arity)]))
+                string.format("expected single mode for %s.%s/%d",
+                    [s(ModuleName), s(ProcName), i(Arity)]))
         )
     ;
         ModeNo = mode_no(N),
@@ -1245,8 +1245,8 @@ lookup_builtin_pred_proc_id(Module, ModuleName, ProcName, PredOrFunc,
             ProcId = ProcId0
         ;
             unexpected($module, $pred,
-                string.format("there is no mode %d for %s/%d",
-                    [i(N), s(ProcName), i(Arity)]))
+                string.format("there is no mode %d for %s.%s/%d",
+                    [i(N), s(ModuleName), s(ProcName), i(Arity)]))
         )
     ).