diff --git a/mers_lib/Cargo.toml b/mers_lib/Cargo.toml
index 1da9484..cbc52fb 100755
--- a/mers_lib/Cargo.toml
+++ b/mers_lib/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "mers_lib"
-version = "0.8.3"
+version = "0.8.4"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 description = "library to use the mers language in other projects"
diff --git a/mers_lib/src/errors/mod.rs b/mers_lib/src/errors/mod.rs
index b39395f..b4642c0 100644
--- a/mers_lib/src/errors/mod.rs
+++ b/mers_lib/src/errors/mod.rs
@@ -276,20 +276,28 @@ impl CheckError {
                             if first_line_nr == last_line_nr {
                                 writeln!(
                                     f,
-                                    "{}Line {first_line_nr} ({}..{}){}",
-                                    indent!(true, false),
-                                    start_with_comments + 1 - first_line_start,
-                                    end_with_comments - last_line_start,
-                                    src_from,
+                                    "{}",
+                                    format!(
+                                        "{}Line {first_line_nr} ({}..{}){}",
+                                        indent!(true, false),
+                                        start_with_comments + 1 - first_line_start,
+                                        end_with_comments - last_line_start,
+                                        src_from,
+                                    )
+                                    .bright_black()
                                 )?;
                             } else {
                                 writeln!(
                                     f,
-                                    "{}Lines {first_line_nr}-{last_line_nr} ({}..{}){}",
-                                    indent!(true, false),
-                                    start_with_comments + 1 - first_line_start,
-                                    end_with_comments - last_line_start,
-                                    src_from,
+                                    "{}",
+                                    format!(
+                                        "{}Lines {first_line_nr}-{last_line_nr} ({}..{}){}",
+                                        indent!(true, false),
+                                        start_with_comments + 1 - first_line_start,
+                                        end_with_comments - last_line_start,
+                                        src_from,
+                                    )
+                                    .bright_black()
                                 )?;
                             }
                             let lines = if cfg.show_comments {
diff --git a/mers_lib/src/program/run/custom_type.rs b/mers_lib/src/program/run/custom_type.rs
index 190b937..3963e2e 100644
--- a/mers_lib/src/program/run/custom_type.rs
+++ b/mers_lib/src/program/run/custom_type.rs
@@ -1,5 +1,7 @@
 use std::{fmt::Debug, sync::Arc};
 
+use colored::Colorize;
+
 use crate::{
     data::{Data, Type},
     errors::{CheckError, SourceRange},
@@ -33,12 +35,25 @@ impl MersStatement for CustomType {
         if init_to.is_some() {
             return Err("can't init to `type` statement".to_string().into());
         }
-        let t = (self.source)(info)?;
-        info.scopes
-            .last_mut()
-            .unwrap()
-            .types
-            .insert(self.name.clone(), t);
+        let t = (self.source)(info);
+        if self.name != "_" {
+            info.scopes
+                .last_mut()
+                .unwrap()
+                .types
+                .insert(self.name.clone(), t?);
+        } else {
+            if let Err(e) = t {
+                return Err(CheckError::new()
+                    .msg(format!(
+                        " {} {} {} (`[[_] := ...]` indicates that `...` must be type-correct)",
+                        "<<".bright_red(),
+                        "Custom type-test failed!".bright_red(),
+                        ">>".bright_red(),
+                    ))
+                    .err(e));
+            }
+        }
         Ok(Type::empty_tuple())
     }
     fn run_custom(&self, _info: &mut Info) -> Data {