diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c2f40c169dc3f..fc0e8f91e2097 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7594,8 +7594,8 @@ namespace ts {
                 }
                 // If the object type is a mapped type { [P in K]: E }, we instantiate E using a mapper that substitutes
                 // the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we construct the
-                // type Box<T[X]>.
-                if (isGenericMappedType(objectType)) {
+                // type Box<T[X]>. Note: this substitution is applied only if K is not and indexing operation iself.
+                if (isGenericMappedType(objectType) && ((<MappedType>objectType).constraintType.flags & TypeFlags.IndexedAccess) === 0) {
                     return getIndexedAccessForMappedType(<MappedType>objectType, indexType, accessNode);
                 }
                 // Otherwise we defer the operation by creating an indexed access type.
@@ -18623,6 +18623,7 @@ namespace ts {
         }
 
         function checkIndexedAccessType(node: IndexedAccessTypeNode) {
+            checkSourceElement(node.objectType);
             checkIndexedAccessIndexType(getTypeFromIndexedAccessTypeNode(node), node);
         }
 
diff --git a/tests/baselines/reference/keyofAndIndexedAccessErrors.errors.txt b/tests/baselines/reference/keyofAndIndexedAccessErrors.errors.txt
index 1e88e2abc4341..0634c3419e014 100644
--- a/tests/baselines/reference/keyofAndIndexedAccessErrors.errors.txt
+++ b/tests/baselines/reference/keyofAndIndexedAccessErrors.errors.txt
@@ -15,6 +15,7 @@ tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(35,21): error
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(36,21): error TS2538: Type 'boolean' cannot be used as an index type.
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(41,31): error TS2538: Type 'boolean' cannot be used as an index type.
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(46,16): error TS2538: Type 'boolean' cannot be used as an index type.
+tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(49,12): error TS1122: A tuple type element list cannot be empty.
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(63,33): error TS2345: Argument of type '"size"' is not assignable to parameter of type '"name" | "width" | "height" | "visible"'.
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(64,33): error TS2345: Argument of type '"name" | "size"' is not assignable to parameter of type '"name" | "width" | "height" | "visible"'.
   Type '"size"' is not assignable to type '"name" | "width" | "height" | "visible"'.
@@ -28,7 +29,7 @@ tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(76,5): error
 tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(77,5): error TS2322: Type 'keyof (T & U)' is not assignable to type 'keyof (T | U)'.
 
 
-==== tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts (24 errors) ====
+==== tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts (25 errors) ====
     class Shape {
         name: string;
         width: number;
@@ -112,6 +113,8 @@ tests/cases/conformance/types/keyof/keyofAndIndexedAccessErrors.ts(77,5): error
     
     type T60 = {}["toString"];
     type T61 = []["toString"];
+               ~~
+!!! error TS1122: A tuple type element list cannot be empty.
     
     declare let cond: boolean;
     
diff --git a/tests/baselines/reference/mappedTypeErrors2.errors.txt b/tests/baselines/reference/mappedTypeErrors2.errors.txt
new file mode 100644
index 0000000000000..f526b5b20a2f9
--- /dev/null
+++ b/tests/baselines/reference/mappedTypeErrors2.errors.txt
@@ -0,0 +1,36 @@
+tests/cases/conformance/types/mapped/mappedTypeErrors2.ts(7,30): error TS2536: Type 'K' cannot be used to index type 'T1<K>'.
+tests/cases/conformance/types/mapped/mappedTypeErrors2.ts(11,30): error TS2536: Type 'K' cannot be used to index type 'T3'.
+tests/cases/conformance/types/mapped/mappedTypeErrors2.ts(14,47): error TS2536: Type 'S' cannot be used to index type 'AB'.
+tests/cases/conformance/types/mapped/mappedTypeErrors2.ts(17,49): error TS2536: Type 'L' cannot be used to index type '{}'.
+
+
+==== tests/cases/conformance/types/mapped/mappedTypeErrors2.ts (4 errors) ====
+    type AB = {
+        a: 'a'
+        b: 'a'
+    }
+    
+    type T1<K extends keyof AB> = { [key in AB[K]]: true }
+    type T2<K extends 'a'|'b'> = T1<K>[K] // BUG 1: should be error for K = 'b'
+                                 ~~~~~~~~
+!!! error TS2536: Type 'K' cannot be used to index type 'T1<K>'.
+    
+    type R = AB[keyof AB]; // "a"
+    type T3 = { [key in R]: true }
+    type T4<K extends 'a'|'b'> = T3[K] // error as expected
+                                 ~~~~~
+!!! error TS2536: Type 'K' cannot be used to index type 'T3'.
+    
+    // BUG 2: 'extra' not checked in AB[S]
+    type T5<S extends 'a'|'b'|'extra'> = {[key in AB[S]]: true}[S]
+                                                  ~~~~~
+!!! error TS2536: Type 'S' cannot be used to index type 'AB'.
+    
+    // Should still error, for L = 'b'
+    type T6<S extends 'a'|'b', L extends 'a'|'b'> = {[key in AB[S]]: true}[L]
+                                                    ~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2536: Type 'L' cannot be used to index type '{}'.
+    
+    // Should be OK
+    type T7<S extends 'a'|'b', L extends 'a'> = {[key in AB[S]]: true}[L]
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeErrors2.js b/tests/baselines/reference/mappedTypeErrors2.js
new file mode 100644
index 0000000000000..42a33d2ff3445
--- /dev/null
+++ b/tests/baselines/reference/mappedTypeErrors2.js
@@ -0,0 +1,49 @@
+//// [mappedTypeErrors2.ts]
+type AB = {
+    a: 'a'
+    b: 'a'
+}
+
+type T1<K extends keyof AB> = { [key in AB[K]]: true }
+type T2<K extends 'a'|'b'> = T1<K>[K] // BUG 1: should be error for K = 'b'
+
+type R = AB[keyof AB]; // "a"
+type T3 = { [key in R]: true }
+type T4<K extends 'a'|'b'> = T3[K] // error as expected
+
+// BUG 2: 'extra' not checked in AB[S]
+type T5<S extends 'a'|'b'|'extra'> = {[key in AB[S]]: true}[S]
+
+// Should still error, for L = 'b'
+type T6<S extends 'a'|'b', L extends 'a'|'b'> = {[key in AB[S]]: true}[L]
+
+// Should be OK
+type T7<S extends 'a'|'b', L extends 'a'> = {[key in AB[S]]: true}[L]
+
+
+//// [mappedTypeErrors2.js]
+
+
+//// [mappedTypeErrors2.d.ts]
+declare type AB = {
+    a: 'a';
+    b: 'a';
+};
+declare type T1<K extends keyof AB> = {
+    [key in AB[K]]: true;
+};
+declare type T2<K extends 'a' | 'b'> = T1<K>[K];
+declare type R = AB[keyof AB];
+declare type T3 = {
+    [key in R]: true;
+};
+declare type T4<K extends 'a' | 'b'> = T3[K];
+declare type T5<S extends 'a' | 'b' | 'extra'> = {
+    [key in AB[S]]: true;
+}[S];
+declare type T6<S extends 'a' | 'b', L extends 'a' | 'b'> = {
+    [key in AB[S]]: true;
+}[L];
+declare type T7<S extends 'a' | 'b', L extends 'a'> = {
+    [key in AB[S]]: true;
+}[L];
diff --git a/tests/cases/conformance/types/mapped/mappedTypeErrors2.ts b/tests/cases/conformance/types/mapped/mappedTypeErrors2.ts
new file mode 100644
index 0000000000000..e02ec1a03a1ca
--- /dev/null
+++ b/tests/cases/conformance/types/mapped/mappedTypeErrors2.ts
@@ -0,0 +1,23 @@
+// @strictNullChecks: true
+// @declaration: true
+
+type AB = {
+    a: 'a'
+    b: 'a'
+}
+
+type T1<K extends keyof AB> = { [key in AB[K]]: true }
+type T2<K extends 'a'|'b'> = T1<K>[K] // BUG 1: should be error for K = 'b'
+
+type R = AB[keyof AB]; // "a"
+type T3 = { [key in R]: true }
+type T4<K extends 'a'|'b'> = T3[K] // error as expected
+
+// BUG 2: 'extra' not checked in AB[S]
+type T5<S extends 'a'|'b'|'extra'> = {[key in AB[S]]: true}[S]
+
+// Should still error, for L = 'b'
+type T6<S extends 'a'|'b', L extends 'a'|'b'> = {[key in AB[S]]: true}[L]
+
+// Should be OK
+type T7<S extends 'a'|'b', L extends 'a'> = {[key in AB[S]]: true}[L]