diff --git a/src/aeso_ast_infer_types.erl b/src/aeso_ast_infer_types.erl
index d52144b..94a9542 100644
--- a/src/aeso_ast_infer_types.erl
+++ b/src/aeso_ast_infer_types.erl
@@ -1241,6 +1241,8 @@ check_type(Env, {constrained_t, Ann, Constraints, Type}, Arity) ->
     TVars = [ Name || {tvar, _, Name} <- extract_typevars(Type) ],
     [ type_error({unused_constraint, C}) || C = {constraint, _, {tvar, _, Name}, _} <- Constraints,
                                             not lists:member(Name, TVars) ],
+    [ type_error({unknown_tvar_constraint, C}) || C = {constraint, _, _, {id, _, Name}} <- Constraints,
+                                                  not lists:member(Name, ["eq", "ord"]) ],
 
     {constrained_t, Ann, Constraints, check_type(Env, Type, Arity)};
 check_type(_Env, {args_t, Ann, Ts}, _) ->
@@ -3422,6 +3424,9 @@ mk_error({type_not_ord, Ann, Type}) ->
 mk_error({unused_constraint, {constraint, Ann, {tvar, _, Name}, _}}) ->
     Msg = io_lib:format("The type variable `~s` is constrained but never used", [Name]),
     mk_t_err(pos(Ann), Msg);
+mk_error({unknown_tvar_constraint, {constraint, _, {tvar, _, TVar}, {id, Ann, C}}}) ->
+    Msg = io_lib:format("Unknown constraint `~s` used on the type variable `~s`", [C, TVar]),
+    mk_t_err(pos(Ann), Msg);
 mk_error(Err) ->
     Msg = io_lib:format("Unknown error: ~p", [Err]),
     mk_t_err(pos(0, 0), Msg).
diff --git a/test/aeso_compiler_tests.erl b/test/aeso_compiler_tests.erl
index a84eeaf..7d630ec 100644
--- a/test/aeso_compiler_tests.erl
+++ b/test/aeso_compiler_tests.erl
@@ -810,83 +810,85 @@ failing_contracts() ->
     , ?TYPE_ERROR(comparable_typevar_constraints,
                   [<<?Pos(21,30)
                      "Values of type `'a` are not comparable by equality">>,
-                   <<?Pos(25,37)
+                   <<?Pos(25,38)
                      "The type variable `'b` is constrained but never used">>,
-                   <<?Pos(56,56)
+                   <<?Pos(29,41)
+                     "Unknown constraint `foo` used on the type variable `'a`">>,
+                   <<?Pos(60,56)
                      "Values of type `address` are not comparable by inequality">>,
-                   <<?Pos(59,58)
+                   <<?Pos(63,58)
                      "Values of type `Chain.ttl` are not comparable by inequality">>,
-                   <<?Pos(62,45)
+                   <<?Pos(66,45)
                      "Values of type `A` are not comparable by inequality">>,
-                   <<?Pos(69,47)
+                   <<?Pos(73,47)
                      "Values of type `(int, char) => bool` are not comparable by inequality">>,
-                   <<?Pos(70,47)
+                   <<?Pos(74,47)
                      "Values of type `(int, char) => bool` are not comparable by equality">>,
-                   <<?Pos(85,71)
+                   <<?Pos(89,71)
                      "Values of type `list(address)` are not comparable by inequality">>,
-                   <<?Pos(88,77)
+                   <<?Pos(92,77)
                      "Values of type `option(address)` are not comparable by inequality">>,
-                   <<?Pos(91,76)
+                   <<?Pos(95,76)
                      "Values of type `(address * int)` are not comparable by inequality">>,
-                   <<?Pos(92,76)
+                   <<?Pos(96,76)
                      "Values of type `(address * int)` are not comparable by equality">>,
-                   <<?Pos(96,68)
+                   <<?Pos(100,68)
                      "Values of type `list((int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(97,68)
+                   <<?Pos(101,68)
                      "Values of type `list((int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(99,74)
+                   <<?Pos(103,74)
                      "Values of type `option((int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(100,74)
+                   <<?Pos(104,74)
                      "Values of type `option((int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(102,73)
+                   <<?Pos(106,73)
                      "Values of type `((int, char) => bool * int)` are not comparable by inequality">>,
-                   <<?Pos(103,73)
+                   <<?Pos(107,73)
                      "Values of type `((int, char) => bool * int)` are not comparable by equality">>,
-                   <<?Pos(107,71)
+                   <<?Pos(111,71)
                      "Values of type `map(int, int)` are not comparable by inequality">>,
-                   <<?Pos(110,80)
+                   <<?Pos(114,80)
                      "Values of type `oracle(int, int)` are not comparable by inequality">>,
-                   <<?Pos(113,98)
+                   <<?Pos(117,98)
                      "Values of type `oracle_query(int, int)` are not comparable by inequality">>,
-                   <<?Pos(116,90)
+                   <<?Pos(120,90)
                      "Values of type `custom_datatype(int)` are not comparable by inequality">>,
-                   <<?Pos(119,84)
+                   <<?Pos(123,84)
                      "Values of type `custom_record(int)` are not comparable by inequality">>,
-                   <<?Pos(124,86)
+                   <<?Pos(128,86)
                      "Values of type `map(address, address)` are not comparable by inequality">>,
-                   <<?Pos(127,95)
+                   <<?Pos(131,95)
                      "Values of type `oracle(address, address)` are not comparable by inequality">>,
-                   <<?Pos(130,113)
+                   <<?Pos(134,113)
                      "Values of type `oracle_query(address, address)` are not comparable by inequality">>,
-                   <<?Pos(133,97)
+                   <<?Pos(137,97)
                      "Values of type `custom_datatype(address)` are not comparable by inequality">>,
-                   <<?Pos(136,91)
+                   <<?Pos(140,91)
                      "Values of type `custom_record(address)` are not comparable by inequality">>,
-                   <<?Pos(141,75)
+                   <<?Pos(145,75)
                      "Values of type `map((int, char) => bool, (int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(142,75)
+                   <<?Pos(146,75)
                      "Values of type `map((int, char) => bool, (int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(144,84)
+                   <<?Pos(148,84)
                      "Values of type `oracle((int, char) => bool, (int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(145,84)
+                   <<?Pos(149,84)
                      "Values of type `oracle((int, char) => bool, (int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(147,102)
+                   <<?Pos(151,102)
                      "Values of type `oracle_query((int, char) => bool, (int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(148,102)
+                   <<?Pos(152,102)
                      "Values of type `oracle_query((int, char) => bool, (int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(150,94)
+                   <<?Pos(154,94)
                      "Values of type `custom_datatype((int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(151,94)
+                   <<?Pos(155,94)
                      "Values of type `custom_datatype((int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(153,88)
+                   <<?Pos(157,88)
                      "Values of type `custom_record((int, char) => bool)` are not comparable by inequality">>,
-                   <<?Pos(154,88)
+                   <<?Pos(158,88)
                      "Values of type `custom_record((int, char) => bool)` are not comparable by equality">>,
-                   <<?Pos(158,35)
+                   <<?Pos(162,35)
                      "Values of type `map(int, int)` are not comparable by inequality">>,
-                   <<?Pos(159,35)
+                   <<?Pos(163,35)
                      "Values of type `('a) => 'a` are not comparable by inequality">>,
-                   <<?Pos(163,34)
+                   <<?Pos(167,34)
                      "Values of type `('b) => 'b` are not comparable by equality">>
                   ])
     , ?TYPE_ERROR(warnings,
diff --git a/test/contracts/comparable_typevar_constraints.aes b/test/contracts/comparable_typevar_constraints.aes
index 11e98fb..852ab5a 100644
--- a/test/contracts/comparable_typevar_constraints.aes
+++ b/test/contracts/comparable_typevar_constraints.aes
@@ -7,12 +7,12 @@ main contract C =
 
     // pass
     function
-        passing_ord: 'a is ord ; ('a, 'a) => bool
+        passing_ord : 'a is ord ; ('a, 'a) => bool
         passing_ord(x, y) = x >= y
 
     // pass
     function
-        passing_eq: 'a is eq ; ('a, 'a) => bool
+        passing_eq : 'a is eq ; ('a, 'a) => bool
         passing_eq(x, y) = x == y
 
     // fail because eq is not specified for 'a
@@ -22,9 +22,13 @@ main contract C =
 
     // fail because 'b is not used
     function
-        fail_unused_tvar: 'a is eq, 'b is eq ; ('a, 'a) => bool
+        fail_unused_tvar : 'a is eq, 'b is eq ; ('a, 'a) => bool
         fail_unused_tvar(x, y) = x == y
 
+    function
+        fail_unknown_constraint : 'a is foo ; ('a) => 'a
+        fail_unknown_constraint(x) = x
+
     // Ord types
 
     function bool_ord(x : bool, y : bool) = x >= y  // pass