diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 128e6410cb432..a965651486b1d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11775,7 +11775,8 @@ namespace ts {
                 const propTypes: Type[] = [];
                 for (const prop of getPropertiesOfType(type)) {
                     if (kind === IndexKind.String || isNumericLiteralName(prop.escapedName)) {
-                        propTypes.push(getTypeOfSymbol(prop));
+                        const propType = getTypeOfSymbol(prop);
+                        propTypes.push(prop.flags & SymbolFlags.Optional ? getTypeWithFacts(propType, TypeFacts.NEUndefined) : propType);
                     }
                 }
                 if (kind === IndexKind.String) {
diff --git a/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.js b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.js
new file mode 100644
index 0000000000000..386b08fbc0047
--- /dev/null
+++ b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.js
@@ -0,0 +1,40 @@
+//// [inferenceOptionalPropertiesToIndexSignatures.ts]
+declare function foo<T>(obj: { [x: string]: T }): T;
+
+declare const x1: { a: string, b: number };
+declare const x2: { a: string, b: number | undefined };
+declare const x3: { a: string, b?: number };
+declare const x4: { a: string, b?: number | undefined };
+
+let a1 = foo(x1);  // string | number
+let a2 = foo(x2);  // string | number | undefined
+let a3 = foo(x3);  // string | number
+let a4 = foo(x4);  // string | number
+
+// Repro from #43045
+
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+
+const obj = {
+    param1: 'value1',
+    ...(param2 ? {param2} : {})
+};
+
+const query = Object.entries(obj).map(
+    ([k, v]) => `${k}=${encodeURIComponent(v)}`
+).join('&');
+
+
+//// [inferenceOptionalPropertiesToIndexSignatures.js]
+"use strict";
+let a1 = foo(x1); // string | number
+let a2 = foo(x2); // string | number | undefined
+let a3 = foo(x3); // string | number
+let a4 = foo(x4); // string | number
+// Repro from #43045
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+const obj = {
+    param1: 'value1',
+    ...(param2 ? { param2 } : {})
+};
+const query = Object.entries(obj).map(([k, v]) => `${k}=${encodeURIComponent(v)}`).join('&');
diff --git a/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.symbols b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.symbols
new file mode 100644
index 0000000000000..9874e516a2b75
--- /dev/null
+++ b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.symbols
@@ -0,0 +1,89 @@
+=== tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts ===
+declare function foo<T>(obj: { [x: string]: T }): T;
+>foo : Symbol(foo, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 0))
+>T : Symbol(T, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 21))
+>obj : Symbol(obj, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 24))
+>x : Symbol(x, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 32))
+>T : Symbol(T, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 21))
+>T : Symbol(T, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 21))
+
+declare const x1: { a: string, b: number };
+>x1 : Symbol(x1, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 2, 13))
+>a : Symbol(a, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 2, 19))
+>b : Symbol(b, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 2, 30))
+
+declare const x2: { a: string, b: number | undefined };
+>x2 : Symbol(x2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 3, 13))
+>a : Symbol(a, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 3, 19))
+>b : Symbol(b, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 3, 30))
+
+declare const x3: { a: string, b?: number };
+>x3 : Symbol(x3, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 4, 13))
+>a : Symbol(a, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 4, 19))
+>b : Symbol(b, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 4, 30))
+
+declare const x4: { a: string, b?: number | undefined };
+>x4 : Symbol(x4, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 5, 13))
+>a : Symbol(a, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 5, 19))
+>b : Symbol(b, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 5, 30))
+
+let a1 = foo(x1);  // string | number
+>a1 : Symbol(a1, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 7, 3))
+>foo : Symbol(foo, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 0))
+>x1 : Symbol(x1, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 2, 13))
+
+let a2 = foo(x2);  // string | number | undefined
+>a2 : Symbol(a2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 8, 3))
+>foo : Symbol(foo, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 0))
+>x2 : Symbol(x2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 3, 13))
+
+let a3 = foo(x3);  // string | number
+>a3 : Symbol(a3, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 9, 3))
+>foo : Symbol(foo, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 0))
+>x3 : Symbol(x3, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 4, 13))
+
+let a4 = foo(x4);  // string | number
+>a4 : Symbol(a4, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 10, 3))
+>foo : Symbol(foo, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 0, 0))
+>x4 : Symbol(x4, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 5, 13))
+
+// Repro from #43045
+
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+>param2 : Symbol(param2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 14, 5))
+>Math.random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+>Math : Symbol(Math, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+>random : Symbol(Math.random, Decl(lib.es5.d.ts, --, --))
+
+const obj = {
+>obj : Symbol(obj, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 16, 5))
+
+    param1: 'value1',
+>param1 : Symbol(param1, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 16, 13))
+
+    ...(param2 ? {param2} : {})
+>param2 : Symbol(param2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 14, 5))
+>param2 : Symbol(param2, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 18, 18))
+
+};
+
+const query = Object.entries(obj).map(
+>query : Symbol(query, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 21, 5))
+>Object.entries(obj).map(    ([k, v]) => `${k}=${encodeURIComponent(v)}`).join : Symbol(Array.join, Decl(lib.es5.d.ts, --, --))
+>Object.entries(obj).map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>Object.entries : Symbol(ObjectConstructor.entries, Decl(lib.es2017.object.d.ts, --, --), Decl(lib.es2017.object.d.ts, --, --))
+>Object : Symbol(Object, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>entries : Symbol(ObjectConstructor.entries, Decl(lib.es2017.object.d.ts, --, --), Decl(lib.es2017.object.d.ts, --, --))
+>obj : Symbol(obj, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 16, 5))
+>map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+
+    ([k, v]) => `${k}=${encodeURIComponent(v)}`
+>k : Symbol(k, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 22, 6))
+>v : Symbol(v, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 22, 8))
+>k : Symbol(k, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 22, 6))
+>encodeURIComponent : Symbol(encodeURIComponent, Decl(lib.es5.d.ts, --, --))
+>v : Symbol(v, Decl(inferenceOptionalPropertiesToIndexSignatures.ts, 22, 8))
+
+).join('&');
+>join : Symbol(Array.join, Decl(lib.es5.d.ts, --, --))
+
diff --git a/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.types b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.types
new file mode 100644
index 0000000000000..b707b156b2ea2
--- /dev/null
+++ b/tests/baselines/reference/inferenceOptionalPropertiesToIndexSignatures.types
@@ -0,0 +1,109 @@
+=== tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts ===
+declare function foo<T>(obj: { [x: string]: T }): T;
+>foo : <T>(obj: { [x: string]: T; }) => T
+>obj : { [x: string]: T; }
+>x : string
+
+declare const x1: { a: string, b: number };
+>x1 : { a: string; b: number; }
+>a : string
+>b : number
+
+declare const x2: { a: string, b: number | undefined };
+>x2 : { a: string; b: number | undefined; }
+>a : string
+>b : number | undefined
+
+declare const x3: { a: string, b?: number };
+>x3 : { a: string; b?: number | undefined; }
+>a : string
+>b : number | undefined
+
+declare const x4: { a: string, b?: number | undefined };
+>x4 : { a: string; b?: number | undefined; }
+>a : string
+>b : number | undefined
+
+let a1 = foo(x1);  // string | number
+>a1 : string | number
+>foo(x1) : string | number
+>foo : <T>(obj: { [x: string]: T; }) => T
+>x1 : { a: string; b: number; }
+
+let a2 = foo(x2);  // string | number | undefined
+>a2 : string | number | undefined
+>foo(x2) : string | number | undefined
+>foo : <T>(obj: { [x: string]: T; }) => T
+>x2 : { a: string; b: number | undefined; }
+
+let a3 = foo(x3);  // string | number
+>a3 : string | number
+>foo(x3) : string | number
+>foo : <T>(obj: { [x: string]: T; }) => T
+>x3 : { a: string; b?: number | undefined; }
+
+let a4 = foo(x4);  // string | number
+>a4 : string | number
+>foo(x4) : string | number
+>foo : <T>(obj: { [x: string]: T; }) => T
+>x4 : { a: string; b?: number | undefined; }
+
+// Repro from #43045
+
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+>param2 : "value2" | null
+>Math.random() < 0.5 ? 'value2' : null : "value2" | null
+>Math.random() < 0.5 : boolean
+>Math.random() : number
+>Math.random : () => number
+>Math : Math
+>random : () => number
+>0.5 : 0.5
+>'value2' : "value2"
+>null : null
+
+const obj = {
+>obj : { param2?: string | undefined; param1: string; }
+>{    param1: 'value1',    ...(param2 ? {param2} : {})} : { param2?: string | undefined; param1: string; }
+
+    param1: 'value1',
+>param1 : string
+>'value1' : "value1"
+
+    ...(param2 ? {param2} : {})
+>(param2 ? {param2} : {}) : { param2: string; } | {}
+>param2 ? {param2} : {} : { param2: string; } | {}
+>param2 : "value2" | null
+>{param2} : { param2: string; }
+>param2 : string
+>{} : {}
+
+};
+
+const query = Object.entries(obj).map(
+>query : string
+>Object.entries(obj).map(    ([k, v]) => `${k}=${encodeURIComponent(v)}`).join('&') : string
+>Object.entries(obj).map(    ([k, v]) => `${k}=${encodeURIComponent(v)}`).join : (separator?: string | undefined) => string
+>Object.entries(obj).map(    ([k, v]) => `${k}=${encodeURIComponent(v)}`) : string[]
+>Object.entries(obj).map : <U>(callbackfn: (value: [string, string], index: number, array: [string, string][]) => U, thisArg?: any) => U[]
+>Object.entries(obj) : [string, string][]
+>Object.entries : { <T>(o: { [s: string]: T; } | ArrayLike<T>): [string, T][]; (o: {}): [string, any][]; }
+>Object : ObjectConstructor
+>entries : { <T>(o: { [s: string]: T; } | ArrayLike<T>): [string, T][]; (o: {}): [string, any][]; }
+>obj : { param2?: string | undefined; param1: string; }
+>map : <U>(callbackfn: (value: [string, string], index: number, array: [string, string][]) => U, thisArg?: any) => U[]
+
+    ([k, v]) => `${k}=${encodeURIComponent(v)}`
+>([k, v]) => `${k}=${encodeURIComponent(v)}` : ([k, v]: [string, string]) => string
+>k : string
+>v : string
+>`${k}=${encodeURIComponent(v)}` : string
+>k : string
+>encodeURIComponent(v) : string
+>encodeURIComponent : (uriComponent: string | number | boolean) => string
+>v : string
+
+).join('&');
+>join : (separator?: string | undefined) => string
+>'&' : "&"
+
diff --git a/tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts b/tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts
new file mode 100644
index 0000000000000..cd675e2facd5d
--- /dev/null
+++ b/tests/cases/compiler/inferenceOptionalPropertiesToIndexSignatures.ts
@@ -0,0 +1,27 @@
+// @strict: true
+// @target: esnext
+
+declare function foo<T>(obj: { [x: string]: T }): T;
+
+declare const x1: { a: string, b: number };
+declare const x2: { a: string, b: number | undefined };
+declare const x3: { a: string, b?: number };
+declare const x4: { a: string, b?: number | undefined };
+
+let a1 = foo(x1);  // string | number
+let a2 = foo(x2);  // string | number | undefined
+let a3 = foo(x3);  // string | number
+let a4 = foo(x4);  // string | number
+
+// Repro from #43045
+
+const param2 = Math.random() < 0.5 ? 'value2' : null;
+
+const obj = {
+    param1: 'value1',
+    ...(param2 ? {param2} : {})
+};
+
+const query = Object.entries(obj).map(
+    ([k, v]) => `${k}=${encodeURIComponent(v)}`
+).join('&');