diff --git a/src/harness/fourslashImpl.ts b/src/harness/fourslashImpl.ts
index 01f8113efe1..eea737837e2 100644
--- a/src/harness/fourslashImpl.ts
+++ b/src/harness/fourslashImpl.ts
@@ -2290,6 +2290,10 @@ namespace FourSlash {
             if (this.testType === FourSlashTestType.Server) {
                 (this.languageService as ts.server.SessionClient).setFormattingOptions(this.formatCodeSettings);
             }
+            else {
+                throw Error(`'setFormatOptions' is not implemented for test type ${formatTestType(this.testType)}.
+Try setting options via compiler options.`);
+            }
             return oldFormatCodeOptions;
         }
 
@@ -4604,4 +4608,17 @@ namespace FourSlash {
         });
         return emTarget;
       }
+
+      function formatTestType(testType: FourSlashTestType): string {
+        switch (testType) {
+            case FourSlashTestType.Native:
+                return "Native";
+            case FourSlashTestType.Shims:
+                return "Shims";
+            case FourSlashTestType.ShimsWithPreprocess:
+                return "ShimsWithPreprocess";
+            case FourSlashTestType.Server:
+                return "Server";
+        }
+      }
 }