diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1c22fc36207df..36b952dd046fa 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4612,8 +4612,8 @@ namespace ts {
                     // the modifiers type is T. Otherwise, the modifiers type is {}.
                     const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
                     const constraint = getConstraintTypeFromMappedType(declaredType);
-                    const extendedConstraint = constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
-                    type.modifiersType = extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
+                    const extendedConstraint = constraint && constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
+                    type.modifiersType = extendedConstraint && extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
                 }
             }
             return type.modifiersType;
diff --git a/tests/baselines/reference/mappedTypeErrors.errors.txt b/tests/baselines/reference/mappedTypeErrors.errors.txt
index f98a11dbaf9be..94644a96af9bd 100644
--- a/tests/baselines/reference/mappedTypeErrors.errors.txt
+++ b/tests/baselines/reference/mappedTypeErrors.errors.txt
@@ -45,9 +45,11 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(130,5): error TS2322: T
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(131,5): error TS2322: Type '{ a: string; }' is not assignable to type '{ [x: string]: any; a?: number | undefined; }'.
   Types of property 'a' are incompatible.
     Type 'string' is not assignable to type 'number | undefined'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(137,16): error TS2322: Type '{}' is not assignable to type 'string'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(137,21): error TS2536: Type 'P' cannot be used to index type 'T'.
 
 
-==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (24 errors) ====
+==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (26 errors) ====
     
     interface Shape {
         name: string;
@@ -249,4 +251,22 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(131,5): error TS2322: T
         ~~
 !!! error TS2322: Type '{ a: string; }' is not assignable to type '{ [x: string]: any; a?: number | undefined; }'.
 !!! error TS2322:   Types of property 'a' are incompatible.
-!!! error TS2322:     Type 'string' is not assignable to type 'number | undefined'.
\ No newline at end of file
+!!! error TS2322:     Type 'string' is not assignable to type 'number | undefined'.
+    
+    // Repro from #13044
+    
+    type Foo2<T, F extends keyof T> = {
+        pf: {[P in F]?: T[P]},
+        pt: {[P in T]?: T[P]}, // note: should be in keyof T
+                   ~
+!!! error TS2322: Type '{}' is not assignable to type 'string'.
+                        ~~~~
+!!! error TS2536: Type 'P' cannot be used to index type 'T'.
+    };
+    type O = {x: number, y: boolean};
+    let o: O = {x: 5, y: false};
+    let f: Foo2<O, 'x'> = {
+        pf: {x: 7},
+        pt: {x: 7, y: false},
+    };
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeErrors.js b/tests/baselines/reference/mappedTypeErrors.js
index 83dc235c7059e..f194b6193fc37 100644
--- a/tests/baselines/reference/mappedTypeErrors.js
+++ b/tests/baselines/reference/mappedTypeErrors.js
@@ -129,7 +129,21 @@ type T2 = { a?: number, [key: string]: any };
 
 let x1: T2 = { a: 'no' };  // Error
 let x2: Partial<T2> = { a: 'no' }; // Error
-let x3: { [P in keyof T2]: T2[P]} = { a: 'no' };  // Error
+let x3: { [P in keyof T2]: T2[P]} = { a: 'no' };  // Error
+
+// Repro from #13044
+
+type Foo2<T, F extends keyof T> = {
+    pf: {[P in F]?: T[P]},
+    pt: {[P in T]?: T[P]}, // note: should be in keyof T
+};
+type O = {x: number, y: boolean};
+let o: O = {x: 5, y: false};
+let f: Foo2<O, 'x'> = {
+    pf: {x: 7},
+    pt: {x: 7, y: false},
+};
+
 
 //// [mappedTypeErrors.js]
 function f1(x) {
@@ -204,6 +218,11 @@ c.setState({ c: true }); // Error
 var x1 = { a: 'no' }; // Error
 var x2 = { a: 'no' }; // Error
 var x3 = { a: 'no' }; // Error
+var o = { x: 5, y: false };
+var f = {
+    pf: { x: 7 },
+    pt: { x: 7, y: false }
+};
 
 
 //// [mappedTypeErrors.d.ts]
@@ -268,3 +287,17 @@ declare let x2: Partial<T2>;
 declare let x3: {
     [P in keyof T2]: T2[P];
 };
+declare type Foo2<T, F extends keyof T> = {
+    pf: {
+        [P in F]?: T[P];
+    };
+    pt: {
+        [P in T]?: T[P];
+    };
+};
+declare type O = {
+    x: number;
+    y: boolean;
+};
+declare let o: O;
+declare let f: Foo2<O, 'x'>;
diff --git a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
index 458dbe9caa412..cec1fbf839554 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
@@ -130,4 +130,17 @@ type T2 = { a?: number, [key: string]: any };
 
 let x1: T2 = { a: 'no' };  // Error
 let x2: Partial<T2> = { a: 'no' }; // Error
-let x3: { [P in keyof T2]: T2[P]} = { a: 'no' };  // Error
\ No newline at end of file
+let x3: { [P in keyof T2]: T2[P]} = { a: 'no' };  // Error
+
+// Repro from #13044
+
+type Foo2<T, F extends keyof T> = {
+    pf: {[P in F]?: T[P]},
+    pt: {[P in T]?: T[P]}, // note: should be in keyof T
+};
+type O = {x: number, y: boolean};
+let o: O = {x: 5, y: false};
+let f: Foo2<O, 'x'> = {
+    pf: {x: 7},
+    pt: {x: 7, y: false},
+};