diff --git a/crates/perry-runtime/src/object/buffer_dispatch.rs b/crates/perry-runtime/src/object/buffer_dispatch.rs
index fba54122af..cda5b577ab 100644
--- a/crates/perry-runtime/src/object/buffer_dispatch.rs
+++ b/crates/perry-runtime/src/object/buffer_dispatch.rs
@@ -1100,7 +1100,7 @@ pub unsafe fn dispatch_buffer_method(
         // non-callable callback) nor iterating. Delegate any method the Buffer
         // API doesn't claim to the shared uint8 typed-array dispatcher (it
         // validates callbacks and reads the typed store); it returns `None` for
-        // names it doesn't implement, preserving the `undefined` fallback.
+        // names it doesn't implement.
         _ => {
             if super::typed_array_proto_thunks::is_typed_array_buffer(addr) {
                 if let Some(r) = super::typed_array_proto_thunks::dispatch_uint8_buffer_method(
@@ -1111,7 +1111,26 @@ pub unsafe fn dispatch_buffer_method(
                     return r;
                 }
             }
-            f64::from_bits(crate::value::TAG_UNDEFINED)
+            // Internal perry hooks (`using` disposal probes and friends) may
+            // reach any receiver; they are duck-typed and must stay non-throwing.
+            if method_name.starts_with("__perry_") {
+                return f64::from_bits(crate::value::TAG_UNDEFINED);
+            }
+            // A method neither the Buffer API nor %TypedArray%.prototype
+            // implements must throw like Node (`buf.charCodeAt is not a
+            // function`), not silently return undefined. The silent fallback
+            // turned a readFileSync-now-returns-Buffer migration into invisible
+            // data corruption downstream: `content.charCodeAt(i)` yielded
+            // undefined in every comparison, so e.g. a NUL-byte binary-file
+            // scan misclassified every text file while the program kept
+            // running as if the calls had worked. Same shape as the string /
+            // number primitive catch-alls, which already route here.
+            crate::error::js_throw_type_error_not_a_function(
+                b"Buffer".as_ptr(),
+                6,
+                method_name.as_ptr(),
+                method_name.len(),
+            )
         }
     }
 }
diff --git a/test-files/test_gap_buffer_unknown_method_throws.ts b/test-files/test_gap_buffer_unknown_method_throws.ts
new file mode 100644
index 0000000000..1b2cd9dc9b
--- /dev/null
+++ b/test-files/test_gap_buffer_unknown_method_throws.ts
@@ -0,0 +1,45 @@
+// Calling a method that neither the Buffer API nor %TypedArray%.prototype
+// implements must throw a TypeError like Node — not silently return undefined.
+//
+// Pre-fix, `dispatch_buffer_method`'s catch-all returned undefined for any
+// unknown name. That silence turned the readFileSync Node-parity migration
+// (no encoding → Buffer, not string) into invisible data corruption for
+// callers still treating the result as a string: `content.charCodeAt(i)`
+// yielded undefined in every comparison and the program kept running as if
+// the calls had worked (real case: an editor's NUL-byte binary-file scan
+// misclassified every text file it opened).
+
+const buf = Buffer.from('hello', 'utf8');
+
+// The implemented Buffer API surface keeps working.
+console.log('toString:', buf.toString('utf8'));
+console.log('indexOf:', buf.indexOf('l'));
+
+// Inherited %TypedArray%.prototype methods keep working via delegation.
+console.log('every:', buf.every((b) => b > 0));
+
+// A String.prototype method reached through a Buffer receiver throws.
+try {
+  (buf as any).charCodeAt(0);
+  console.log('charCodeAt: no throw');
+} catch (e) {
+  console.log('charCodeAt throws TypeError:', e instanceof TypeError);
+  const msg = (e as Error).message;
+  console.log('mentions method:', msg.includes('charCodeAt'));
+  console.log('mentions not-a-function:', msg.includes('is not a function'));
+}
+
+// A method that exists nowhere throws too.
+try {
+  (buf as any).definitelyNotAMethod(1, 2);
+  console.log('bogus: no throw');
+} catch (e) {
+  console.log('bogus throws TypeError:', e instanceof TypeError);
+  console.log(
+    'mentions not-a-function:',
+    (e as Error).message.includes('is not a function')
+  );
+}
+
+// The throw is catchable and execution continues normally afterwards.
+console.log('still running:', buf.readUInt8(0));