diff --git a/huia-compiler/src/context/mod.rs b/huia-compiler/src/context/mod.rs
index 9464adb..30b8b7f 100644
--- a/huia-compiler/src/context/mod.rs
+++ b/huia-compiler/src/context/mod.rs
@@ -329,6 +329,10 @@ impl Context {
         Context::new("Test context")
     }
 
+    pub fn types(&self) -> Vec<&Ty> {
+        self.types.iter().collect()
+    }
+
     /// Create a type variable of unknown type.
     ///
     /// Unresolved types should be removed by typechecking and any remaining
diff --git a/huia-compiler/src/error.rs b/huia-compiler/src/error.rs
index 1bdec55..2f95334 100644
--- a/huia-compiler/src/error.rs
+++ b/huia-compiler/src/error.rs
@@ -50,4 +50,5 @@ pub enum ErrorKind {
     UnknownVariable,
     InconsistentBranchTypes,
     TypeInferenceFailure,
+    UnknownType,
 }
diff --git a/huia-compiler/src/improvements/mod.rs b/huia-compiler/src/improvements/mod.rs
index 018d79b..5e433d8 100644
--- a/huia-compiler/src/improvements/mod.rs
+++ b/huia-compiler/src/improvements/mod.rs
@@ -3,4 +3,5 @@ pub mod clause_return_types;
 pub mod desugar_infix;
 pub mod desugar_unary;
 pub mod infix_folding;
+pub mod no_unresolved_types_remaining;
 pub mod unary_folding;
diff --git a/huia-compiler/src/improvements/no_unresolved_types_remaining.rs b/huia-compiler/src/improvements/no_unresolved_types_remaining.rs
new file mode 100644
index 0000000..4ffe256
--- /dev/null
+++ b/huia-compiler/src/improvements/no_unresolved_types_remaining.rs
@@ -0,0 +1,21 @@
+use crate::context::Context;
+use crate::error::ErrorKind;
+
+pub fn pass(context: &mut Context) {
+    let mut unresolved_types = Vec::default();
+    let all_types = context.types();
+    for (idx, ty) in all_types.iter().enumerate() {
+        if ty.is_unresolved() {
+            let name = match ty.name() {
+                Some(name_idx) => context.get_string(name_idx).unwrap().to_string(),
+                None => format!("T{:0>4}", idx),
+            };
+            let message = format!("Unable to resolve type {}", name);
+            let location = ty.get_location().unwrap().clone();
+            unresolved_types.push((message, location));
+        }
+    }
+    for (message, location) in unresolved_types {
+        context.compile_error(&message, location, ErrorKind::UnknownType);
+    }
+}
diff --git a/huia-compiler/src/lib.rs b/huia-compiler/src/lib.rs
index 6582403..f8404b6 100644
--- a/huia-compiler/src/lib.rs
+++ b/huia-compiler/src/lib.rs
@@ -16,7 +16,8 @@ use huia_parser::ast::Term;
 use std::fs;
 
 pub fn compile_file(path: &str) -> Context {
-    let contents = fs::read_to_string(path).expect("Unable to open file");
+    let contents = fs::read_to_string(path)
+        .unwrap_or_else(|e| panic!("Unable to open file '{}': {}", path, e));
     let terms = Term::file(&contents).expect("Unable to parse file");
     let mut context = Context::new(path);
     let mut builder = ir::builder::Builder::default();
@@ -42,5 +43,7 @@ pub fn compile_file(path: &str) -> Context {
         improvements::clause_return_types::pass(&mut method, &mut context);
     });
 
+    improvements::no_unresolved_types_remaining::pass(&mut context);
+
     context
 }