diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e772c2f0057..4fa3dcf560c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -22701,13 +22701,18 @@ namespace ts {
                     // want to say that number is not an array type. But if the input was just
                     // number and string input is allowed, we want to say that number is not an
                     // array type or a string type.
+                    const isIterable = !!getIteratedTypeOfIterable(inputType, /* errorNode */ undefined, allowAsyncIterables, /*allowSyncIterables*/ true, checkAssignability);
                     const diagnostic = !allowStringInput || hasStringConstituent
                         ? downlevelIteration
                             ? Diagnostics.Type_0_is_not_an_array_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator
-                            : Diagnostics.Type_0_is_not_an_array_type
+                            : isIterable
+                                ? Diagnostics.Type_0_is_not_an_array_type_Use_compiler_option_downlevelIteration_to_allow_iterating_of_iterators
+                                : Diagnostics.Type_0_is_not_an_array_type
                         : downlevelIteration
                             ? Diagnostics.Type_0_is_not_an_array_type_or_a_string_type_or_does_not_have_a_Symbol_iterator_method_that_returns_an_iterator
-                            : Diagnostics.Type_0_is_not_an_array_type_or_a_string_type;
+                            : isIterable
+                                ? Diagnostics.Type_0_is_not_an_array_type_or_a_string_type_Use_compiler_option_downlevelIteration_to_allow_iterating_of_iterators
+                                : Diagnostics.Type_0_is_not_an_array_type_or_a_string_type;
                     error(errorNode, diagnostic, typeToString(arrayType));
                 }
                 return hasStringConstituent ? stringType : undefined;
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 2e3377b08eb..bbc00e6fbc6 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1992,6 +1992,14 @@
         "category": "Error",
         "code": 2567
     },
+    "Type '{0}' is not an array type. Use compiler option '--downlevelIteration' to allow iterating of iterators.": {
+        "category": "Error",
+        "code": 2568
+    },
+    "Type '{0}' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.": {
+        "category": "Error",
+        "code": 2569
+    },
 
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck10.errors.txt b/tests/baselines/reference/ES5For-ofTypeCheck10.errors.txt
index e608378d7f1..6bb1d505680 100644
--- a/tests/baselines/reference/ES5For-ofTypeCheck10.errors.txt
+++ b/tests/baselines/reference/ES5For-ofTypeCheck10.errors.txt
@@ -1,5 +1,5 @@
 tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck10.ts(9,6): error TS2304: Cannot find name 'Symbol'.
-tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck10.ts(14,15): error TS2495: Type 'StringIterator' is not an array type or a string type.
+tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck10.ts(14,15): error TS2569: Type 'StringIterator' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
 
 
 ==== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck10.ts (2 errors) ====
@@ -20,4 +20,4 @@ tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck10.ts(14,1
     
     for (var v of new StringIterator) { }
                   ~~~~~~~~~~~~~~~~~~
-!!! error TS2495: Type 'StringIterator' is not an array type or a string type.
\ No newline at end of file
+!!! error TS2569: Type 'StringIterator' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
\ No newline at end of file
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck13.errors.txt b/tests/baselines/reference/ES5For-ofTypeCheck13.errors.txt
new file mode 100644
index 00000000000..b64781a4873
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck13.errors.txt
@@ -0,0 +1,10 @@
+tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts(4,19): error TS2569: Type 'Set<string>' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
+
+
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts (1 errors) ====
+    const strSet: Set<string> = new Set()
+    strSet.add('Hello')
+    strSet.add('World')
+    for (const str of strSet) { }
+                      ~~~~~~
+!!! error TS2569: Type 'Set<string>' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
\ No newline at end of file
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck13.js b/tests/baselines/reference/ES5For-ofTypeCheck13.js
new file mode 100644
index 00000000000..216aa57f97c
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck13.js
@@ -0,0 +1,13 @@
+//// [ES5For-ofTypeCheck13.ts]
+const strSet: Set<string> = new Set()
+strSet.add('Hello')
+strSet.add('World')
+for (const str of strSet) { }
+
+//// [ES5For-ofTypeCheck13.js]
+var strSet = new Set();
+strSet.add('Hello');
+strSet.add('World');
+for (var _i = 0, strSet_1 = strSet; _i < strSet_1.length; _i++) {
+    var str = strSet_1[_i];
+}
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck13.symbols b/tests/baselines/reference/ES5For-ofTypeCheck13.symbols
new file mode 100644
index 00000000000..14a1fe161a5
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck13.symbols
@@ -0,0 +1,20 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts ===
+const strSet: Set<string> = new Set()
+>strSet : Symbol(strSet, Decl(ES5For-ofTypeCheck13.ts, 0, 5))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+strSet.add('Hello')
+>strSet.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>strSet : Symbol(strSet, Decl(ES5For-ofTypeCheck13.ts, 0, 5))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+
+strSet.add('World')
+>strSet.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>strSet : Symbol(strSet, Decl(ES5For-ofTypeCheck13.ts, 0, 5))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+
+for (const str of strSet) { }
+>str : Symbol(str, Decl(ES5For-ofTypeCheck13.ts, 3, 10))
+>strSet : Symbol(strSet, Decl(ES5For-ofTypeCheck13.ts, 0, 5))
+
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck13.types b/tests/baselines/reference/ES5For-ofTypeCheck13.types
new file mode 100644
index 00000000000..4812ae8d82c
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck13.types
@@ -0,0 +1,25 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts ===
+const strSet: Set<string> = new Set()
+>strSet : Set<string>
+>Set : Set<T>
+>new Set() : Set<any>
+>Set : SetConstructor
+
+strSet.add('Hello')
+>strSet.add('Hello') : Set<string>
+>strSet.add : (value: string) => Set<string>
+>strSet : Set<string>
+>add : (value: string) => Set<string>
+>'Hello' : "Hello"
+
+strSet.add('World')
+>strSet.add('World') : Set<string>
+>strSet.add : (value: string) => Set<string>
+>strSet : Set<string>
+>add : (value: string) => Set<string>
+>'World' : "World"
+
+for (const str of strSet) { }
+>str : any
+>strSet : Set<string>
+
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck14.errors.txt b/tests/baselines/reference/ES5For-ofTypeCheck14.errors.txt
new file mode 100644
index 00000000000..3a03491f110
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck14.errors.txt
@@ -0,0 +1,8 @@
+tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts(2,17): error TS2568: Type 'Set<number>' is not an array type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
+
+
+==== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts (1 errors) ====
+    var union: string | Set<number>
+    for (const e of union) { }
+                    ~~~~~
+!!! error TS2568: Type 'Set<number>' is not an array type. Use compiler option '--downlevelIteration' to allow iterating of iterators.
\ No newline at end of file
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck14.js b/tests/baselines/reference/ES5For-ofTypeCheck14.js
new file mode 100644
index 00000000000..87e55c41a0e
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck14.js
@@ -0,0 +1,9 @@
+//// [ES5For-ofTypeCheck14.ts]
+var union: string | Set<number>
+for (const e of union) { }
+
+//// [ES5For-ofTypeCheck14.js]
+var union;
+for (var _i = 0, union_1 = union; _i < union_1.length; _i++) {
+    var e = union_1[_i];
+}
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck14.symbols b/tests/baselines/reference/ES5For-ofTypeCheck14.symbols
new file mode 100644
index 00000000000..efa7d296d9d
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck14.symbols
@@ -0,0 +1,9 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts ===
+var union: string | Set<number>
+>union : Symbol(union, Decl(ES5For-ofTypeCheck14.ts, 0, 3))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+for (const e of union) { }
+>e : Symbol(e, Decl(ES5For-ofTypeCheck14.ts, 1, 10))
+>union : Symbol(union, Decl(ES5For-ofTypeCheck14.ts, 0, 3))
+
diff --git a/tests/baselines/reference/ES5For-ofTypeCheck14.types b/tests/baselines/reference/ES5For-ofTypeCheck14.types
new file mode 100644
index 00000000000..03a822316ac
--- /dev/null
+++ b/tests/baselines/reference/ES5For-ofTypeCheck14.types
@@ -0,0 +1,9 @@
+=== tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts ===
+var union: string | Set<number>
+>union : string | Set<number>
+>Set : Set<T>
+
+for (const e of union) { }
+>e : string
+>union : string | Set<number>
+
diff --git a/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts b/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts
new file mode 100644
index 00000000000..e6748b9373b
--- /dev/null
+++ b/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck13.ts
@@ -0,0 +1,6 @@
+//@target: ES5
+//@lib: ES6
+const strSet: Set<string> = new Set()
+strSet.add('Hello')
+strSet.add('World')
+for (const str of strSet) { }
\ No newline at end of file
diff --git a/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts b/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts
new file mode 100644
index 00000000000..9474fcec5d9
--- /dev/null
+++ b/tests/cases/conformance/statements/for-ofStatements/ES5For-ofTypeCheck14.ts
@@ -0,0 +1,4 @@
+//@target: ES5
+//@lib: ES6
+var union: string | Set<number>
+for (const e of union) { }
\ No newline at end of file