diff --git a/src/format_csharp.js b/src/format_csharp.js
index 9282da217b8..ae91deaf12d 100644
--- a/src/format_csharp.js
+++ b/src/format_csharp.js
@@ -127,6 +127,7 @@ class CSharpFormatter {
     const text = type.name;
     switch (text) {
       case 'any': return `[object]${optionalSuffix}`;
+      case 'unknown': return `[object]${optionalSuffix}`;
       case 'Array': return `[IEnumerable]${optionalSuffix}`;
       case 'float': return `[float]${optionalSuffix}`;
       case 'function': {
diff --git a/src/format_java.js b/src/format_java.js
index c50a3afe883..ea9bc5a5d53 100644
--- a/src/format_java.js
+++ b/src/format_java.js
@@ -163,6 +163,7 @@ class JavaFormatter {
       case 'Readable': return '[InputStream]';
       case 'Serializable': return '[Object]';
       case 'URL': return '[String]';
+      case 'unknown': return '[Object]';
     }
     return `[${text}]`;
   }
diff --git a/src/format_python.js b/src/format_python.js
index 3b1cdd2fb8a..aa38acf6da3 100644
--- a/src/format_python.js
+++ b/src/format_python.js
@@ -114,6 +114,7 @@ class PythonFormatter {
       case 'long': return '[int]';
       case 'Buffer': return '[bytes]';
       case 'Serializable': return '[Dict]';
+      case 'unknown': return '[Any]';
     }
     return `[${text}]`;
   }