diff --git a/src/fun/check/mod.rs b/src/fun/check/mod.rs
index 7a210bd4..e54bdbd3 100644
--- a/src/fun/check/mod.rs
+++ b/src/fun/check/mod.rs
@@ -1,3 +1,4 @@
 pub mod set_entrypoint;
 pub mod shared_names;
+pub mod unbound_refs;
 pub mod unbound_vars;
diff --git a/src/fun/check/unbound_refs.rs b/src/fun/check/unbound_refs.rs
new file mode 100644
index 00000000..74f8d480
--- /dev/null
+++ b/src/fun/check/unbound_refs.rs
@@ -0,0 +1,37 @@
+use crate::{
+  diagnostics::Diagnostics,
+  fun::{Ctx, Definitions, Name, Term},
+  maybe_grow,
+};
+use std::collections::HashSet;
+
+impl Ctx<'_> {
+  pub fn check_unbound_refs(&mut self) -> Result<(), Diagnostics> {
+    self.info.start_pass();
+    for def in self.book.defs.values() {
+      let mut unbounds = HashSet::new();
+      for rule in def.rules.iter() {
+        rule.body.check_unbound_refs(&self.book.defs, &mut unbounds);
+      }
+      for unbound in unbounds {
+        self.info.add_rule_error(format!("Reference to undefined function '{unbound}'"), def.name.clone());
+      }
+    }
+    self.info.fatal(())
+  }
+}
+
+impl Term {
+  pub fn check_unbound_refs(&self, defs: &Definitions, unbounds: &mut HashSet<Name>) {
+    maybe_grow(|| {
+      if let Term::Ref { nam } = self {
+        if !defs.contains_key(nam) {
+          unbounds.insert(nam.clone());
+        }
+      }
+      for child in self.children() {
+        child.check_unbound_refs(defs, unbounds);
+      }
+    })
+  }
+}
diff --git a/src/fun/mod.rs b/src/fun/mod.rs
index 60ccaf46..164295d0 100644
--- a/src/fun/mod.rs
+++ b/src/fun/mod.rs
@@ -36,7 +36,7 @@ impl Ctx<'_> {
 #[derive(Debug, Clone, Default)]
 pub struct Book {
   /// The function definitions.
-  pub defs: IndexMap<Name, Definition>,
+  pub defs: Definitions,
 
   /// The algebraic datatypes defined by the program
   pub adts: Adts,
@@ -48,6 +48,7 @@ pub struct Book {
   pub entrypoint: Option<Name>,
 }
 
+pub type Definitions = IndexMap<Name, Definition>;
 pub type Adts = IndexMap<Name, Adt>;
 pub type Constructors = IndexMap<Name, Name>;
 
diff --git a/src/lib.rs b/src/lib.rs
index d2fbf23b..b6900e01 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -140,6 +140,8 @@ pub fn desugar_book(
     ctx.book.float_combinators(MAX_NET_SIZE);
   }
 
+  ctx.check_unbound_refs()?;
+
   ctx.prune(opts.prune);
 
   if opts.merge {
diff --git a/tests/golden_tests/hangs/bad_dup_interaction.bend b/tests/golden_tests/hangs/bad_dup_interaction.bend
new file mode 100644
index 00000000..21a3d669
--- /dev/null
+++ b/tests/golden_tests/hangs/bad_dup_interaction.bend
@@ -0,0 +1,11 @@
+# A net that expands inifinitely like church exponentiation
+# Compiles to this net:
+# a
+# & ({(b c) (d b)} (c d)) ~ {(e a) e}
+main =
+  let {a1 a2} =
+    @b
+    let {b1 b2} = b
+    let (c1, c2) = b2
+    ((b1 c2), c1)
+  (a1 a2)
\ No newline at end of file
diff --git a/tests/golden_tests/run_file/unbound_wrap.bend b/tests/golden_tests/run_file/unbound_wrap.bend
new file mode 100644
index 00000000..b21d4a79
--- /dev/null
+++ b/tests/golden_tests/run_file/unbound_wrap.bend
@@ -0,0 +1,10 @@
+type Maybe = (Some x) | None
+
+Maybe/bind val nxt = match val {
+  Maybe/Some: (nxt val.x)
+  Maybe/None: Maybe/None
+}
+
+main = with Maybe {
+  (wrap 1)
+}
\ No newline at end of file
diff --git a/tests/snapshots/run_file__unbound_wrap.bend.snap b/tests/snapshots/run_file__unbound_wrap.bend.snap
new file mode 100644
index 00000000..a154a25f
--- /dev/null
+++ b/tests/snapshots/run_file__unbound_wrap.bend.snap
@@ -0,0 +1,7 @@
+---
+source: tests/golden_tests.rs
+input_file: tests/golden_tests/run_file/unbound_wrap.bend
+---
+Errors:
+In definition 'main':
+  Reference to undefined function 'Maybe/wrap'