diff --git a/implementations/rust/preserves-schema/src/compiler/context.rs b/implementations/rust/preserves-schema/src/compiler/context.rs
index 874a917..3f22563 100644
--- a/implementations/rust/preserves-schema/src/compiler/context.rs
+++ b/implementations/rust/preserves-schema/src/compiler/context.rs
@@ -199,8 +199,12 @@ impl<'a, 'm> FunctionContext<'a, 'm> {
         return item(seq!["Err", parens![self.conformance_err_code()]]);
     }
 
+    pub fn fully_qualified_error_context(&self) -> String {
+        self.m.module_path.0.join(".") + "." + &self.error_context
+    }
+
     pub fn conformance_err_code(&self) -> Item {
         return item(seq!["_support::ParseError::conformance_error", parens![
-            escape_string(&(self.m.module_path.0.join(".") + "." + &self.error_context))]]);
+            escape_string(&self.fully_qualified_error_context())]]);
     }
 }
diff --git a/implementations/rust/preserves-schema/src/compiler/unparsers.rs b/implementations/rust/preserves-schema/src/compiler/unparsers.rs
index 23478a0..dfcb040 100644
--- a/implementations/rust/preserves-schema/src/compiler/unparsers.rs
+++ b/implementations/rust/preserves-schema/src/compiler/unparsers.rs
@@ -1,7 +1,7 @@
 use crate::*;
 use crate::gen::schema::*;
 use crate::syntax::block::constructors::*;
-use crate::syntax::block::{Emittable, Item};
+use crate::syntax::block::{Emittable, Item, escape_string};
 
 use std::cell::Cell;
 use std::rc::Rc;
@@ -82,8 +82,11 @@ pub fn gen_definition_unparser(m: &mut ModuleContext, n: &str, d: &Definition) {
                     ps.extend(pattern_n);
                     let (patpat, vc) = destruct(&mut ctxt, item(names::render_constructor(n)), true, &record_type(&ps));
                     body.push(item(seq!["let ", patpat, " = value;"]));
-                    body.push(item(seq!["preserves::value::merge(vec!", brackets(ps.iter().map(
-                        |p| named_pattern_unparser(&mut ctxt, p, &vc)).collect()), ")"]));
+                    body.push(item(seq![
+                        "preserves::value::merge(vec!", brackets(ps.iter().map(
+                            |p| named_pattern_unparser(&mut ctxt, p, &vc)).collect()
+                        ), ").expect", parens![escape_string(&(
+                            "merge of ".to_owned() + &ctxt.fully_qualified_error_context()))]]));
                 }
                 Definition::Pattern(p) => {
                     let (patpat, vc) = destruct(&mut ctxt, item(names::render_constructor(n)), true, &pattern_type(p));