diff --git a/.changeset/fast-rings-tie.md b/.changeset/fast-rings-tie.md
new file mode 100644
index 0000000..03f68ae
--- /dev/null
+++ b/.changeset/fast-rings-tie.md
@@ -0,0 +1,5 @@
+---
+"@zazuko/env": patch
+---
+
+`Dataset#serialize` would fail with some parsers when unrecognized prefix was used
diff --git a/packages/env/lib/Dataset.ts b/packages/env/lib/Dataset.ts
index 8b6c6d3..ecb753c 100644
--- a/packages/env/lib/Dataset.ts
+++ b/packages/env/lib/Dataset.ts
@@ -105,7 +105,11 @@ export function createConstructor(env: Environment<FormatsFactory>): DatasetCtor
             return { ...map, [prefix[0]]: prefix[1] }
           }
 
-          return { ...map, [prefix]: knownPrefixes[prefix] }
+          if (prefix in knownPrefixes) {
+            return { ...map, [prefix]: knownPrefixes[prefix] }
+          }
+
+          return map
         }, {}),
       }))
     }