diff --git a/src/server/session.ts b/src/server/session.ts
index dc0d4f42f5c79..b2c72a32b8b51 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -1367,7 +1367,7 @@ namespace ts.server {
                 emptyArray;
         }
 
-        private getSyntacticDiagnosticsSync(args: protocol.SyntacticDiagnosticsSyncRequestArgs): readonly protocol.Diagnostic[] | readonly protocol.DiagnosticWithLinePosition[] {
+        private getSyntacticDiagnosticsSync(args: protocol.SyntacticDiagnosticsSyncRequestArgs) {
             const { configFile } = this.getConfigFileAndProject(args);
             if (configFile) {
                 // all the config file errors are reported as part of semantic check so nothing to report here
@@ -1377,7 +1377,7 @@ namespace ts.server {
             return this.getDiagnosticsWorker(args, /*isSemantic*/ false, (project, file) => project.getLanguageService().getSyntacticDiagnostics(file), !!args.includeLinePosition);
         }
 
-        private getSemanticDiagnosticsSync(args: protocol.SemanticDiagnosticsSyncRequestArgs): readonly protocol.Diagnostic[] | readonly protocol.DiagnosticWithLinePosition[] {
+        private getSemanticDiagnosticsSync(args: protocol.SemanticDiagnosticsSyncRequestArgs) {
             const { configFile, project } = this.getConfigFileAndProject(args);
             if (configFile) {
                 return this.getConfigFileDiagnostics(configFile, project!, !!args.includeLinePosition); // TODO: GH#18217
@@ -1385,7 +1385,7 @@ namespace ts.server {
             return this.getDiagnosticsWorker(args, /*isSemantic*/ true, (project, file) => project.getLanguageService().getSemanticDiagnostics(file).filter(d => !!d.file), !!args.includeLinePosition);
         }
 
-        private getSuggestionDiagnosticsSync(args: protocol.SuggestionDiagnosticsSyncRequestArgs): readonly protocol.Diagnostic[] | readonly protocol.DiagnosticWithLinePosition[] {
+        private getSuggestionDiagnosticsSync(args: protocol.SuggestionDiagnosticsSyncRequestArgs) {
             const { configFile } = this.getConfigFileAndProject(args);
             if (configFile) {
                 // Currently there are no info diagnostics for config files.
@@ -2198,7 +2198,25 @@ namespace ts.server {
             const scriptInfo = project.getScriptInfoForNormalizedPath(file)!;
             const { startPosition, endPosition } = this.getStartAndEndPosition(args, scriptInfo);
 
-            const codeActions = project.getLanguageService().getCodeFixesAtPosition(file, startPosition, endPosition, args.errorCodes, this.getFormatOptions(file), this.getPreferences(file));
+            let codeActions: readonly CodeFixAction[];
+            try {
+                codeActions = project.getLanguageService().getCodeFixesAtPosition(file, startPosition, endPosition, args.errorCodes, this.getFormatOptions(file), this.getPreferences(file));
+            }
+            catch(e) {
+                const ls = project.getLanguageService();
+                const existingDiagCodes = [
+                    ...ls.getSyntacticDiagnostics(file),
+                    ...ls.getSemanticDiagnostics(file),
+                    ...ls.getSuggestionDiagnostics(file)
+                ].map(d =>
+                    decodedTextSpanIntersectsWith(startPosition, endPosition - startPosition, d.start!, d.length!)
+                    && d.code);
+                const badCode = args.errorCodes.find(c => !existingDiagCodes.includes(c));
+                if (badCode !== undefined) {
+                    e.message = `BADCLIENT: Bad error code, ${badCode} not found in range ${startPosition}..${endPosition} (found: ${existingDiagCodes.join(", ")}); could have caused this error:\n${e.message}`;
+                }
+                throw e;
+            }
             return simplifiedResult ? codeActions.map(codeAction => this.mapCodeFixAction(codeAction)) : codeActions;
         }
 
diff --git a/src/services/codefixes/fixUnusedIdentifier.ts b/src/services/codefixes/fixUnusedIdentifier.ts
index db7e0550e9a38..9c755729c20e8 100644
--- a/src/services/codefixes/fixUnusedIdentifier.ts
+++ b/src/services/codefixes/fixUnusedIdentifier.ts
@@ -237,8 +237,10 @@ namespace ts.codefix {
         if (isParameter(parent)) {
             tryDeleteParameter(changes, sourceFile, parent, checker, sourceFiles, program, cancellationToken, isFixAll);
         }
-        else if (!isFixAll || !(isIdentifier(token) && FindAllReferences.Core.isSymbolReferencedInFile(token, checker, sourceFile))) {
-            changes.delete(sourceFile, isImportClause(parent) ? token : isComputedPropertyName(parent) ? parent.parent : parent);
+        else if (!(isFixAll && isIdentifier(token) && FindAllReferences.Core.isSymbolReferencedInFile(token, checker, sourceFile))) {
+            const node = isImportClause(parent) ? token : isComputedPropertyName(parent) ? parent.parent : parent;
+            Debug.assert(node !== sourceFile, "should not delete whole source file");
+            changes.delete(sourceFile, node);
         }
     }
 
diff --git a/src/services/textChanges.ts b/src/services/textChanges.ts
index adbcfe68bef42..2425ae43d1271 100644
--- a/src/services/textChanges.ts
+++ b/src/services/textChanges.ts
@@ -1361,7 +1361,11 @@ namespace ts.textChanges {
                     break;
 
                 default:
-                    if (isImportClause(node.parent) && node.parent.name === node) {
+                    if (!node.parent) {
+                        // a misbehaving client can reach here with the SourceFile node
+                        deleteNode(changes, sourceFile, node);
+                    }
+                    else if (isImportClause(node.parent) && node.parent.name === node) {
                         deleteDefaultImport(changes, sourceFile, node.parent);
                     }
                     else if (isCallExpression(node.parent) && contains(node.parent.arguments, node)) {