diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 46cb8bb4937..5b1d4d77bf7 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14599,9 +14599,13 @@ namespace ts {
             }
             const prop = getPropertyOfType(apparentType, right.escapedText);
             if (!prop) {
-                const stringIndexType = getIndexTypeOfType(apparentType, IndexKind.String);
-                if (stringIndexType) {
-                    return stringIndexType;
+                const indexInfo = getIndexInfoOfType(apparentType, IndexKind.String);
+                if (indexInfo && indexInfo.type) {
+                    if (indexInfo.isReadonly && (isAssignmentTarget(node) || isDeleteTarget(node))) {
+                        error(node, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(apparentType));
+                        return unknownType;
+                    }
+                    return indexInfo.type;
                 }
                 if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
                     reportNonexistentProperty(right, type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType ? apparentType : type);