diff --git a/crates/automation/src/documents.rs b/crates/automation/src/documents.rs
index 6e2c0a83f5..40aa4ac522 100644
--- a/crates/automation/src/documents.rs
+++ b/crates/automation/src/documents.rs
@@ -320,6 +320,10 @@ impl Automation {
         if let Some(mono) = a.get("mono") {
             let unknown = |c: &Value| ToolError::InvalidArgs(format!("unknown mono compression {c} (ccitt_g4, retain)"));
             let mono = mono.as_object().ok_or_else(|| ToolError::InvalidArgs("mono must be an object: { compression: ccitt_g4|retain }".into()))?;
+            // A misspelled key would otherwise leave the preset's setting in place unnoticed.
+            if let Some(key) = mono.keys().find(|k| *k != "compression") {
+                return Err(ToolError::InvalidArgs(format!("unknown mono key {key:?} (compression)")));
+            }
             if let Some(c) = mono.get("compression") {
                 settings.mono = match c.as_str() {
                     Some("ccitt_g4" | "ccitt" | "g4") => MonoCompression::CcittG4,
diff --git a/crates/automation/tests/it/automation.rs b/crates/automation/tests/it/automation.rs
index 8392b6722f..dcd321eddb 100644
--- a/crates/automation/tests/it/automation.rs
+++ b/crates/automation/tests/it/automation.rs
@@ -6455,9 +6455,9 @@ fn doc_optimize_stores_black_and_white_images_as_ccitt_group_4() {
         })
         .collect();
     assert_eq!(filters, [b"CCITTFaxDecode".to_vec()]);
-    // An unknown compression, one that isn't a name, or a mono that isn't an object is refused,
-    // not read as the default.
-    for mono in [json!({ "compression": "jbig2" }), json!({ "compression": false }), json!("retain")] {
+    // An unknown compression, one that isn't a name, a misspelled key, or a mono that isn't an
+    // object is refused, not read as the default.
+    for mono in [json!({ "compression": "jbig2" }), json!({ "compression": false }), json!({ "compresson": "retain" }), json!("retain")] {
         assert!(matches!(a.call("doc_optimize", &json!({ "doc": doc, "path": "x.pdf", "mono": mono })), Err(ToolError::InvalidArgs(_))), "{mono}");
     }
 }