diff --git a/src/diagnostics.ts b/src/diagnostics.ts
index bf8b3059e..944fa1796 100644
--- a/src/diagnostics.ts
+++ b/src/diagnostics.ts
@@ -2,7 +2,7 @@
 import * as fs from "fs"
 import chalk from "chalk"
 import { Syntax } from "./ast";
-import {format, MapLike, FormatArg, countDigits, mapValues, prettyPrint, assert} from "./util";
+import {format, MapLike, FormatArg, countDigits, mapValues, prettyPrint, assert, isPlainObject, isPrimitive, JsonObject, Json} from "./util";
 import { BOLT_DIAG_NUM_EXTRA_LINES } from "./constants";
 import { TextPos, TextFile, TextSpan } from "./text";
 
@@ -125,7 +125,7 @@ export class DiagnosticPrinter {
       out += chalk.bold.yellow(`${span.file.origPath}:${span.start.line}:${span.start.column}: `);
     }
     if (diagnostic.args !== undefined) {
-      out += format(diagnostic.message, mapValues(diagnostic.args, prettyPrint)) + '\n';
+      out += format(diagnostic.message, diagnostic.args) + '\n';
     } else {
       out += diagnostic.message + '\n';
     }
diff --git a/src/frontend.ts b/src/frontend.ts
index b342348ef..3dcf5e877 100644
--- a/src/frontend.ts
+++ b/src/frontend.ts
@@ -19,7 +19,7 @@ import { TypeChecker } from "./types"
 import { CheckInvalidFilePaths, CheckTypeAssignments, CheckReferences } from "./checks"
 import { SymbolResolver, BoltSymbolResolutionStrategy } from "./resolver"
 import { Evaluator } from "./evaluator"
-import { getNodeLanguage, ParseError, ScanError } from "./common"
+import { getNodeLanguage, ParseError, ScanError, describeKind } from "./common"
 import { Package, loadPackageMetadata } from "./package"
 import { TextFile } from "./text"
 import { Scanner } from "./scanner"
@@ -191,7 +191,7 @@ export class Frontend {
       } else if (e instanceof ParseError) {
         this.diagnostics.add({
           message: E_PARSE_ERROR,
-          args: { actual: kindToString(e.actual.kind), expected: e.expected.map(kindToString) },
+          args: { actual: describeKind(e.actual.kind), expected: e.expected.map(describeKind) },
           node: e.actual,
           severity: 'fatal',
         });
diff --git a/src/util.ts b/src/util.ts
index feab105e7..e0bc22951 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -627,6 +627,9 @@ type FormatModifierFn = (value: any) => any;
 const FORMAT_MODIFIERS: MapLike<FormatModifierFn> = {
   enum(elements) {
     return enumOr(elements);
+  },
+  pretty(value) {
+    return value.map(prettyPrint);
   }
 }
 
@@ -643,17 +646,20 @@ export function format(message: string, data: MapLike<FormatArg>) {
     switch (mode) {
       case FormatScanMode.ScanningParamModifier:
         if (ch === '}') {
-          modifiers.push(modifierName);
-          push();
-        } else if (ch === ':') {
           if (modifierName.length === 0) {
             throw new Error(`Parameter modfifier name in format string is empty.`)
           }
           modifiers.push(modifierName);
-          modifierName = '';
+          push();
+        } else if (ch === ':') {
+          if (modifierName.length > 0) {
+            modifiers.push(modifierName);
+            modifierName = '';
+          }
         } else {
           modifierName += ch;
         }
+        break;
       case FormatScanMode.ScanningParamName:
         if (ch === '}') {
           push();