diff --git a/compiler/mono/src/borrow.rs b/compiler/mono/src/borrow.rs
index 5a5261b6a5..419158288a 100644
--- a/compiler/mono/src/borrow.rs
+++ b/compiler/mono/src/borrow.rs
@@ -46,7 +46,14 @@ pub fn infer_borrow<'a>(
     // component (in top-sorted order, from primitives (std-lib) to main)
 
     let successor_map = &make_successor_mapping(arena, procs);
-    let successors = move |key: &Symbol| successor_map[key].iter().copied();
+    let successors = move |key: &Symbol| {
+        let slice = match successor_map.get(key) {
+            None => &[] as &[_],
+            Some(s) => s.as_slice(),
+        };
+
+        slice.iter().copied()
+    };
 
     let mut symbols = Vec::with_capacity_in(procs.len(), arena);
     symbols.extend(procs.keys().map(|x| x.0));
@@ -217,7 +224,10 @@ impl<'a> DeclarationToIndex<'a> {
                 }
             }
         }
-        unreachable!("symbol/layout combo must be in DeclarationToIndex")
+        unreachable!(
+            "symbol/layout {:?} {:?} combo must be in DeclarationToIndex",
+            needle_symbol, needle_layout
+        )
     }
 }
 
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index a39785e684..5fb8473a77 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -6159,6 +6159,38 @@ fn build_call<'a>(
     Stmt::Let(assigned, Expr::Call(call), return_layout, hole)
 }
 
+/// See https://github.com/rtfeldman/roc/issues/1549
+///
+/// What happened is that a function has a type error, but the arguments are not processed.
+/// That means specializations were missing. Normally that is not a problem, but because
+/// of our closure strategy, internal functions can "leak". That's what happened here.
+///
+/// The solution is to evaluate the arguments as normal, and only when calling the function give an error
+fn evaluate_arguments_then_runtime_error<'a>(
+    env: &mut Env<'a, '_>,
+    procs: &mut Procs<'a>,
+    layout_cache: &mut LayoutCache<'a>,
+    msg: String,
+    loc_args: std::vec::Vec<(Variable, Located<roc_can::expr::Expr>)>,
+) -> Stmt<'a> {
+    let arena = env.arena;
+
+    // eventually we will throw this runtime error
+    let result = Stmt::RuntimeError(env.arena.alloc(msg));
+
+    // but, we also still evaluate and specialize the arguments to give better error messages
+    let arg_symbols = Vec::from_iter_in(
+        loc_args
+            .iter()
+            .map(|(_, arg_expr)| possible_reuse_symbol(env, procs, &arg_expr.value)),
+        arena,
+    )
+    .into_bump_slice();
+
+    let iter = loc_args.into_iter().rev().zip(arg_symbols.iter().rev());
+    assign_to_symbols(env, procs, layout_cache, iter, result)
+}
+
 #[allow(clippy::too_many_arguments)]
 fn call_by_name<'a>(
     env: &mut Env<'a, '_>,
@@ -6177,14 +6209,16 @@ fn call_by_name<'a>(
                 "Hit an unresolved type variable {:?} when creating a layout for {:?} (var {:?})",
                 var, proc_name, fn_var
             );
-            Stmt::RuntimeError(env.arena.alloc(msg))
+
+            evaluate_arguments_then_runtime_error(env, procs, layout_cache, msg, loc_args)
         }
         Err(LayoutProblem::Erroneous) => {
             let msg = format!(
                 "Hit an erroneous type when creating a layout for {:?}",
                 proc_name
             );
-            Stmt::RuntimeError(env.arena.alloc(msg))
+
+            evaluate_arguments_then_runtime_error(env, procs, layout_cache, msg, loc_args)
         }
         Ok(RawFunctionLayout::Function(arg_layouts, lambda_set, ret_layout)) => {
             if procs.module_thunks.contains(&proc_name) {
diff --git a/compiler/types/src/subs.rs b/compiler/types/src/subs.rs
index 4aca1a9eb5..6625ab0630 100644
--- a/compiler/types/src/subs.rs
+++ b/compiler/types/src/subs.rs
@@ -327,6 +327,14 @@ fn subs_fmt_desc(this: &Descriptor, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
     write!(f, " m: {:?}", &this.mark)
 }
 
+pub struct SubsFmtContent<'a>(pub &'a Content, pub &'a Subs);
+
+impl<'a> fmt::Debug for SubsFmtContent<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        subs_fmt_content(self.0, self.1, f)
+    }
+}
+
 fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt::Result {
     match this {
         Content::FlexVar(name) => write!(f, "Flex({:?})", name),
@@ -345,6 +353,14 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
     }
 }
 
+pub struct SubsFmtFlatType<'a>(pub &'a FlatType, pub &'a Subs);
+
+impl<'a> fmt::Debug for SubsFmtFlatType<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        subs_fmt_flat_type(self.0, self.1, f)
+    }
+}
+
 fn subs_fmt_flat_type(this: &FlatType, subs: &Subs, f: &mut fmt::Formatter) -> fmt::Result {
     match this {
         FlatType::Apply(name, arguments) => {
diff --git a/compiler/unify/src/unify.rs b/compiler/unify/src/unify.rs
index 169559476d..51771efe07 100644
--- a/compiler/unify/src/unify.rs
+++ b/compiler/unify/src/unify.rs
@@ -1076,8 +1076,8 @@ fn unify_flat_type(
             // any other combination is a mismatch
             mismatch!(
                 "Trying to unify two flat types that are incompatible: {:?} ~ {:?}",
-                other1,
-                other2
+                roc_types::subs::SubsFmtFlatType(other1, subs),
+                roc_types::subs::SubsFmtFlatType(other2, subs)
             )
         }
     }