diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3350a4f08fb..e516a6319ec 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -17651,8 +17651,8 @@ namespace ts {
                         if (flags & NodeFlags.Readonly) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "readonly");
                         }
-                        else if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature && node.kind !== SyntaxKind.IndexSignature &&
-                            !(node.kind == SyntaxKind.Parameter && isParameterPropertyDeclaration(<ParameterDeclaration> node))) {
+                        else if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature && node.kind !== SyntaxKind.IndexSignature && node.kind !== SyntaxKind.Parameter) {
+                            // If node.kind === SyntaxKind.Parameter, checkParameter report an error if it's not a parameter property.
                             return grammarErrorOnNode(modifier, Diagnostics.readonly_modifier_can_only_appear_on_a_property_declaration_or_index_signature);
                         }
                         flags |= NodeFlags.Readonly;
diff --git a/tests/baselines/reference/readonlyInAmbientClass.errors.txt b/tests/baselines/reference/readonlyInAmbientClass.errors.txt
index 303f04038f3..844e8b58ef8 100644
--- a/tests/baselines/reference/readonlyInAmbientClass.errors.txt
+++ b/tests/baselines/reference/readonlyInAmbientClass.errors.txt
@@ -1,9 +1,13 @@
 tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts(2,14): error TS2369: A parameter property is only allowed in a constructor implementation.
+tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts(3,9): error TS2369: A parameter property is only allowed in a constructor implementation.
 
 
-==== tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts (1 errors) ====
+==== tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts (2 errors) ====
     declare class C{
     	constructor(readonly x: number);
     	            ~~~~~~~~~~~~~~~~~~
+!!! error TS2369: A parameter property is only allowed in a constructor implementation.
+    	method(readonly x: number);
+    	       ~~~~~~~~~~~~~~~~~~
 !!! error TS2369: A parameter property is only allowed in a constructor implementation.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/readonlyInAmbientClass.js b/tests/baselines/reference/readonlyInAmbientClass.js
index 9be7c1702c9..0e199b5ca4f 100644
--- a/tests/baselines/reference/readonlyInAmbientClass.js
+++ b/tests/baselines/reference/readonlyInAmbientClass.js
@@ -1,6 +1,7 @@
 //// [readonlyInAmbientClass.ts]
 declare class C{
 	constructor(readonly x: number);
+	method(readonly x: number);
 }
 
 //// [readonlyInAmbientClass.js]
diff --git a/tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts b/tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts
index ca710532dd3..c84594eb4f0 100644
--- a/tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts
+++ b/tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyInAmbientClass.ts
@@ -1,3 +1,4 @@
 declare class C{
 	constructor(readonly x: number);
+	method(readonly x: number);
 }
\ No newline at end of file