diff --git a/compiler/parse_type_defn.m b/compiler/parse_type_defn.m
index c75c98e13..2f1bc3050 100644
--- a/compiler/parse_type_defn.m
+++ b/compiler/parse_type_defn.m
@@ -379,6 +379,17 @@ parse_maybe_exist_quant_constructor(ModuleName, VarSet, Ordinal, Term,
 
 parse_constructor(ModuleName, VarSet, Ordinal, ExistQVars, Term,
         MaybeConstructor) :-
+    ( if Term = term.functor(term.atom(":"), [_, _], ColonContext) then
+        ColonNameArity = name_arity(":", 2),
+        Pieces = [words("Error: attempt to define a data constructor"),
+            words("named"), name_arity(ColonNameArity), suffix(","),
+            words("which is a sign that a typo has replaced the")] ++
+            color_as_correct([words("semicolon")]) ++
+            [words("between two data constructors with a")] ++
+            color_as_incorrect([words("colon.")]) ++ [nl],
+        Spec = spec($pred, severity_error, phase_t2pt, ColonContext, Pieces),
+        MaybeConstructor = error1(one_or_more(Spec, []))
+    else
         get_existential_constraints_from_term(ModuleName, VarSet, Term,
             BeforeConstraintsTerm, MaybeConstraints),
         (
@@ -391,7 +402,8 @@ parse_constructor(ModuleName, VarSet, Ordinal, ExistQVars, Term,
                 % the constructor are ignored. This is to allow you to define
                 % ';'/2 and 'some'/2 constructors.
                 % XXX I (zs) don't think that this is a good idea; I think such
-            % constructors are much more likely to be confusing than useful.
+                % constructors are much more likely to be confusing
+                % than useful.
                 BeforeConstraintsTerm = term.functor(term.atom("{}"),
                     [InsideBracesTerm], _Context)
             then
@@ -399,7 +411,8 @@ parse_constructor(ModuleName, VarSet, Ordinal, ExistQVars, Term,
             else
                 MainTerm = BeforeConstraintsTerm
             ),
-        ContextPieces = cord.singleton(words("In constructor definition:")),
+            ContextPieces =
+                cord.singleton(words("In constructor definition:")),
             parse_implicitly_qualified_sym_name_and_args(ModuleName, VarSet,
                 ContextPieces, MainTerm, MaybeFunctorAndArgTerms),
             (
@@ -492,6 +505,7 @@ parse_constructor(ModuleName, VarSet, Ordinal, ExistQVars, Term,
                 det_list_to_one_or_more(Specs, OoMSpecs),
                 MaybeConstructor = error1(OoMSpecs)
             )
+        )
     ).
 
 %---------------------%
diff --git a/tests/invalid/Mmakefile b/tests/invalid/Mmakefile
index 567a41dbd..0561f3702 100644
--- a/tests/invalid/Mmakefile
+++ b/tests/invalid/Mmakefile
@@ -157,6 +157,7 @@ BORING_SINGLEMODULE_PROGS = \
 	det_errors_and_io \
 	det_errors_cc \
 	det_errors_deep \
+	du_type_with_colon \
 	duplicate_mutable \
 	ee_invalid \
 	erroneous_throw_promise \
diff --git a/tests/invalid/du_type_with_colon.err_exp b/tests/invalid/du_type_with_colon.err_exp
new file mode 100644
index 000000000..2bc87b1eb
--- /dev/null
+++ b/tests/invalid/du_type_with_colon.err_exp
@@ -0,0 +1,8 @@
+du_type_with_colon.m:023: Error: attempt to define a data constructor named
+du_type_with_colon.m:023:   `:'/2, which is a sign that a typo has replaced the
+du_type_with_colon.m:023:   semicolon between two data constructors with a
+du_type_with_colon.m:023:   colon.
+du_type_with_colon.m:028: Error: attempt to define a data constructor named
+du_type_with_colon.m:028:   `:'/2, which is a sign that a typo has replaced the
+du_type_with_colon.m:028:   semicolon between two data constructors with a
+du_type_with_colon.m:028:   colon.
diff --git a/tests/invalid/du_type_with_colon.m b/tests/invalid/du_type_with_colon.m
new file mode 100644
index 000000000..a36642721
--- /dev/null
+++ b/tests/invalid/du_type_with_colon.m
@@ -0,0 +1,32 @@
+%---------------------------------------------------------------------------%
+% vim: ts=4 sw=4 et ft=mercury
+%---------------------------------------------------------------------------%
+%
+% The first typo below used to generate this confusing error message:
+%
+% du_type_with_colon.m:010: In the first argument of function symbol `:' of
+% du_type_with_colon.m:010:   the type `foo'/0:
+% du_type_with_colon.m:010:   error: the type `foo_a'/0 is undefined.
+% du_type_with_colon.m:010:   (Did you mean `foo'?)
+% du_type_with_colon.m:011: In the second argument of function symbol `:' of
+% du_type_with_colon.m:011:   the type `foo'/0:
+% du_type_with_colon.m:011:   error: the type `foo_b'/0 is undefined.
+% du_type_with_colon.m:011:   (Did you mean `foo'?)
+%
+
+:- module du_type_with_colon.
+
+:- interface.
+
+:- type foo
+    --->    foo_a
+    :       foo_b.      % This colon should be a SEMIcolon.
+
+:- type bar
+    --->    bar_a
+    :       bar_b       % This colon should be a SEMIcolon.
+    :       bar_c.      % This colon should be a SEMIcolon.
+
+% Export something to avoid a warning we don't want to test.
+:- type dummy
+    --->    dummy.