diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c5ffd98f9ae..54339bbdf3f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10458,7 +10458,7 @@ namespace ts {
                         existingProp.keyType = getUnionType([existingProp.keyType, keyType]);
                     }
                     else {
-                        const modifiersProp = getPropertyOfType(modifiersType, propName);
+                        const modifiersProp = isTypeUsableAsPropertyName(keyType) ? getPropertyOfType(modifiersType, getPropertyNameFromType(keyType)) : undefined;
                         const isOptional = !!(templateModifiers & MappedTypeModifiers.IncludeOptional ||
                             !(templateModifiers & MappedTypeModifiers.ExcludeOptional) && modifiersProp && modifiersProp.flags & SymbolFlags.Optional);
                         const isReadonly = !!(templateModifiers & MappedTypeModifiers.IncludeReadonly ||
diff --git a/tests/baselines/reference/mappedTypeAsClauses.js b/tests/baselines/reference/mappedTypeAsClauses.js
index 75c1d32cd60..9bb361db932 100644
--- a/tests/baselines/reference/mappedTypeAsClauses.js
+++ b/tests/baselines/reference/mappedTypeAsClauses.js
@@ -28,6 +28,20 @@ type DoubleProp<T> = { [P in keyof T & string as `${P}1` | `${P}2`]: T[P] }
 type TD1 = DoubleProp<{ a: string, b: number }>;  // { a1: string, a2: string, b1: number, b2: number }
 type TD2 = keyof TD1;  // 'a1' | 'a2' | 'b1' | 'b2'
 type TD3<U> = keyof DoubleProp<U>;  // `${keyof U & string}1` | `${keyof U & string}2`
+
+// Repro from #40619
+
+type Lazyify<T> = {
+    [K in keyof T as `get${capitalize string & K}`]: () => T[K]
+};
+
+interface Person {
+    readonly name: string;
+    age: number;
+    location?: string;
+}
+
+type LazyPerson = Lazyify<Person>;
 
 
 //// [mappedTypeAsClauses.js]
@@ -82,3 +96,12 @@ declare type TD1 = DoubleProp<{
 }>;
 declare type TD2 = keyof TD1;
 declare type TD3<U> = keyof DoubleProp<U>;
+declare type Lazyify<T> = {
+    [K in keyof T as `get${capitalize string & K}`]: () => T[K];
+};
+interface Person {
+    readonly name: string;
+    age: number;
+    location?: string;
+}
+declare type LazyPerson = Lazyify<Person>;
diff --git a/tests/baselines/reference/mappedTypeAsClauses.symbols b/tests/baselines/reference/mappedTypeAsClauses.symbols
index 75768ce31af..31a9ca0bef8 100644
--- a/tests/baselines/reference/mappedTypeAsClauses.symbols
+++ b/tests/baselines/reference/mappedTypeAsClauses.symbols
@@ -108,3 +108,36 @@ type TD3<U> = keyof DoubleProp<U>;  // `${keyof U & string}1` | `${keyof U & str
 >DoubleProp : Symbol(DoubleProp, Decl(mappedTypeAsClauses.ts, 21, 85))
 >U : Symbol(U, Decl(mappedTypeAsClauses.ts, 28, 9))
 
+// Repro from #40619
+
+type Lazyify<T> = {
+>Lazyify : Symbol(Lazyify, Decl(mappedTypeAsClauses.ts, 28, 34))
+>T : Symbol(T, Decl(mappedTypeAsClauses.ts, 32, 13))
+
+    [K in keyof T as `get${capitalize string & K}`]: () => T[K]
+>K : Symbol(K, Decl(mappedTypeAsClauses.ts, 33, 5))
+>T : Symbol(T, Decl(mappedTypeAsClauses.ts, 32, 13))
+>K : Symbol(K, Decl(mappedTypeAsClauses.ts, 33, 5))
+>T : Symbol(T, Decl(mappedTypeAsClauses.ts, 32, 13))
+>K : Symbol(K, Decl(mappedTypeAsClauses.ts, 33, 5))
+
+};
+
+interface Person {
+>Person : Symbol(Person, Decl(mappedTypeAsClauses.ts, 34, 2))
+
+    readonly name: string;
+>name : Symbol(Person.name, Decl(mappedTypeAsClauses.ts, 36, 18))
+
+    age: number;
+>age : Symbol(Person.age, Decl(mappedTypeAsClauses.ts, 37, 26))
+
+    location?: string;
+>location : Symbol(Person.location, Decl(mappedTypeAsClauses.ts, 38, 16))
+}
+
+type LazyPerson = Lazyify<Person>;
+>LazyPerson : Symbol(LazyPerson, Decl(mappedTypeAsClauses.ts, 40, 1))
+>Lazyify : Symbol(Lazyify, Decl(mappedTypeAsClauses.ts, 28, 34))
+>Person : Symbol(Person, Decl(mappedTypeAsClauses.ts, 34, 2))
+
diff --git a/tests/baselines/reference/mappedTypeAsClauses.types b/tests/baselines/reference/mappedTypeAsClauses.types
index 438d9c1fcbf..f2426a9c74c 100644
--- a/tests/baselines/reference/mappedTypeAsClauses.types
+++ b/tests/baselines/reference/mappedTypeAsClauses.types
@@ -66,3 +66,25 @@ type TD2 = keyof TD1;  // 'a1' | 'a2' | 'b1' | 'b2'
 type TD3<U> = keyof DoubleProp<U>;  // `${keyof U & string}1` | `${keyof U & string}2`
 >TD3 : `${keyof U & string}1` | `${keyof U & string}2`
 
+// Repro from #40619
+
+type Lazyify<T> = {
+>Lazyify : Lazyify<T>
+
+    [K in keyof T as `get${capitalize string & K}`]: () => T[K]
+};
+
+interface Person {
+    readonly name: string;
+>name : string
+
+    age: number;
+>age : number
+
+    location?: string;
+>location : string | undefined
+}
+
+type LazyPerson = Lazyify<Person>;
+>LazyPerson : Lazyify<Person>
+
diff --git a/tests/cases/conformance/types/mapped/mappedTypeAsClauses.ts b/tests/cases/conformance/types/mapped/mappedTypeAsClauses.ts
index 51f6bd720bf..542503e82dc 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeAsClauses.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeAsClauses.ts
@@ -30,3 +30,17 @@ type DoubleProp<T> = { [P in keyof T & string as `${P}1` | `${P}2`]: T[P] }
 type TD1 = DoubleProp<{ a: string, b: number }>;  // { a1: string, a2: string, b1: number, b2: number }
 type TD2 = keyof TD1;  // 'a1' | 'a2' | 'b1' | 'b2'
 type TD3<U> = keyof DoubleProp<U>;  // `${keyof U & string}1` | `${keyof U & string}2`
+
+// Repro from #40619
+
+type Lazyify<T> = {
+    [K in keyof T as `get${capitalize string & K}`]: () => T[K]
+};
+
+interface Person {
+    readonly name: string;
+    age: number;
+    location?: string;
+}
+
+type LazyPerson = Lazyify<Person>;