diff --git a/include/xo/reader/define_xs.hpp b/include/xo/reader/define_xs.hpp
index 88e07237..d3ca7b39 100644
--- a/include/xo/reader/define_xs.hpp
+++ b/include/xo/reader/define_xs.hpp
@@ -38,6 +38,7 @@ namespace xo {
             virtual void on_typedescr(TypeDescr td,
                                       exprstatestack * /*p_stack*/,
                                       rp<Expression> * /*p_emit_expr*/) override;
+            virtual void on_colon(exprstatestack * p_stack) override;
 
         private:
             /**
diff --git a/src/reader/define_xs.cpp b/src/reader/define_xs.cpp
index f1ce7669..7c0ed3bd 100644
--- a/src/reader/define_xs.cpp
+++ b/src/reader/define_xs.cpp
@@ -423,6 +423,30 @@ namespace xo {
             }
         }
 
+        void
+        define_xs::on_colon(exprstatestack * p_stack) {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * self_name = "exprstate::on_colon";
+
+            /* lots of illegal states */
+            if (!this->admits_colon())
+            {
+                throw std::runtime_error(tostr(self_name,
+                                               ": unexpected colon for parsing state",
+                                               xtag("state", *this)));
+            }
+
+            if (this->exs_type_ == exprstatetype::def_1) {
+                this->exs_type_ = exprstatetype::def_2;
+
+                p_stack->push_exprstate(exprstate::expect_type());
+            } else {
+                assert(false);
+            }
+        }
+
     } /*namespace scm*/
 } /*namespace xo*/
 
diff --git a/src/reader/exprstate.cpp b/src/reader/exprstate.cpp
index 2b4b777e..d4079646 100644
--- a/src/reader/exprstate.cpp
+++ b/src/reader/exprstate.cpp
@@ -595,7 +595,7 @@ namespace xo {
         }
 
         void
-        exprstate::on_colon(exprstatestack * p_stack) {
+        exprstate::on_colon(exprstatestack * /*p_stack*/) {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
 
@@ -609,13 +609,7 @@ namespace xo {
                                                xtag("state", *this)));
             }
 
-            if (this->exs_type_ == exprstatetype::def_1) {
-                this->exs_type_ = exprstatetype::def_2;
-
-                p_stack->push_exprstate(exprstate::expect_type());
-            } else {
-                assert(false);
-            }
+            assert(false);
         }
 
         void