diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c17893cebe823..2eeea77e49268 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4715,6 +4715,10 @@ namespace ts {
             return prop ? getTypeOfSymbol(prop) : undefined;
         }
 
+        function getTypeOfPropertyOrIndexSignature(type: Type, name: __String): Type {
+            return getTypeOfPropertyOfType(type, name) || isNumericLiteralName(name) && getIndexTypeOfType(type, IndexKind.Number) || getIndexTypeOfType(type, IndexKind.String) || unknownType;
+        }
+
         function isTypeAny(type: Type | undefined) {
             return type && (type.flags & TypeFlags.Any) !== 0;
         }
@@ -15657,7 +15661,7 @@ namespace ts {
                 }
                 const propType = getTypeOfPropertyOfType(type, propName);
                 const narrowedPropType = propType && narrowType(propType);
-                return propType === narrowedPropType ? type : filterType(type, t => isTypeComparableTo(getTypeOfPropertyOfType(t, propName)!, narrowedPropType!));
+                return propType === narrowedPropType ? type : filterType(type, t => isTypeComparableTo(getTypeOfPropertyOrIndexSignature(t, propName), narrowedPropType!));
             }
 
             function narrowTypeByTruthiness(type: Type, expr: Expression, assumeTrue: boolean): Type {
diff --git a/tests/baselines/reference/discriminatedUnionTypes2.errors.txt b/tests/baselines/reference/discriminatedUnionTypes2.errors.txt
index c50f7a2767aac..15ed6968aeac2 100644
--- a/tests/baselines/reference/discriminatedUnionTypes2.errors.txt
+++ b/tests/baselines/reference/discriminatedUnionTypes2.errors.txt
@@ -83,4 +83,26 @@ tests/cases/conformance/types/union/discriminatedUnionTypes2.ts(32,11): error TS
             const data: null = carrier.data
         }
     }
+    
+    // Repro from #28935
+    
+    type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+    
+    function f30(foo: Foo) {
+        if (foo.tag) {
+            foo;
+        }
+        else {
+            foo;
+        }
+    }
+    
+    function f31(foo: Foo) {
+        if (foo.tag === true) {
+            foo;
+        }
+        else {
+            foo;
+        }
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/discriminatedUnionTypes2.js b/tests/baselines/reference/discriminatedUnionTypes2.js
index e2b5d99ddcd84..ec5b2a6b5f13f 100644
--- a/tests/baselines/reference/discriminatedUnionTypes2.js
+++ b/tests/baselines/reference/discriminatedUnionTypes2.js
@@ -71,6 +71,28 @@ function f20<Data>(carrier: DataCarrier<Data>) {
         const data: null = carrier.data
     }
 }
+
+// Repro from #28935
+
+type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+
+function f30(foo: Foo) {
+    if (foo.tag) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
+
+function f31(foo: Foo) {
+    if (foo.tag === true) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
 
 
 //// [discriminatedUnionTypes2.js]
@@ -126,3 +148,19 @@ function f20(carrier) {
         var data = carrier.data;
     }
 }
+function f30(foo) {
+    if (foo.tag) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
+function f31(foo) {
+    if (foo.tag === true) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
diff --git a/tests/baselines/reference/discriminatedUnionTypes2.symbols b/tests/baselines/reference/discriminatedUnionTypes2.symbols
index 38cfcb0b23ab4..682c62b9be545 100644
--- a/tests/baselines/reference/discriminatedUnionTypes2.symbols
+++ b/tests/baselines/reference/discriminatedUnionTypes2.symbols
@@ -225,3 +225,51 @@ function f20<Data>(carrier: DataCarrier<Data>) {
     }
 }
 
+// Repro from #28935
+
+type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+>Foo : Symbol(Foo, Decl(discriminatedUnionTypes2.ts, 71, 1))
+>tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 12))
+>x : Symbol(x, Decl(discriminatedUnionTypes2.ts, 75, 23))
+>tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 39))
+>y : Symbol(y, Decl(discriminatedUnionTypes2.ts, 75, 51))
+>x : Symbol(x, Decl(discriminatedUnionTypes2.ts, 75, 69))
+
+function f30(foo: Foo) {
+>f30 : Symbol(f30, Decl(discriminatedUnionTypes2.ts, 75, 90))
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 77, 13))
+>Foo : Symbol(Foo, Decl(discriminatedUnionTypes2.ts, 71, 1))
+
+    if (foo.tag) {
+>foo.tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 12), Decl(discriminatedUnionTypes2.ts, 75, 39))
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 77, 13))
+>tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 12), Decl(discriminatedUnionTypes2.ts, 75, 39))
+
+        foo;
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 77, 13))
+    }
+    else {
+        foo;
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 77, 13))
+    }
+}
+
+function f31(foo: Foo) {
+>f31 : Symbol(f31, Decl(discriminatedUnionTypes2.ts, 84, 1))
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 86, 13))
+>Foo : Symbol(Foo, Decl(discriminatedUnionTypes2.ts, 71, 1))
+
+    if (foo.tag === true) {
+>foo.tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 12), Decl(discriminatedUnionTypes2.ts, 75, 39))
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 86, 13))
+>tag : Symbol(tag, Decl(discriminatedUnionTypes2.ts, 75, 12), Decl(discriminatedUnionTypes2.ts, 75, 39))
+
+        foo;
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 86, 13))
+    }
+    else {
+        foo;
+>foo : Symbol(foo, Decl(discriminatedUnionTypes2.ts, 86, 13))
+    }
+}
+
diff --git a/tests/baselines/reference/discriminatedUnionTypes2.types b/tests/baselines/reference/discriminatedUnionTypes2.types
index b654fc1a78d5c..58819d0d3495a 100644
--- a/tests/baselines/reference/discriminatedUnionTypes2.types
+++ b/tests/baselines/reference/discriminatedUnionTypes2.types
@@ -239,3 +239,53 @@ function f20<Data>(carrier: DataCarrier<Data>) {
     }
 }
 
+// Repro from #28935
+
+type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+>Foo : Foo
+>tag : true
+>true : true
+>x : number
+>tag : false
+>false : false
+>y : number
+>x : string
+
+function f30(foo: Foo) {
+>f30 : (foo: Foo) => void
+>foo : Foo
+
+    if (foo.tag) {
+>foo.tag : string | boolean
+>foo : Foo
+>tag : string | boolean
+
+        foo;
+>foo : { tag: true; x: number; } | { [x: string]: string; }
+    }
+    else {
+        foo;
+>foo : { tag: false; y: number; } | { [x: string]: string; }
+    }
+}
+
+function f31(foo: Foo) {
+>f31 : (foo: Foo) => void
+>foo : Foo
+
+    if (foo.tag === true) {
+>foo.tag === true : boolean
+>foo.tag : string | boolean
+>foo : Foo
+>tag : string | boolean
+>true : true
+
+        foo;
+>foo : { tag: true; x: number; }
+    }
+    else {
+        foo;
+>foo : { tag: false; y: number; } | { [x: string]: string; }
+    }
+}
+
diff --git a/tests/cases/conformance/types/union/discriminatedUnionTypes2.ts b/tests/cases/conformance/types/union/discriminatedUnionTypes2.ts
index 789bc263c7ebc..a11e1447780ee 100644
--- a/tests/cases/conformance/types/union/discriminatedUnionTypes2.ts
+++ b/tests/cases/conformance/types/union/discriminatedUnionTypes2.ts
@@ -72,3 +72,25 @@ function f20<Data>(carrier: DataCarrier<Data>) {
         const data: null = carrier.data
     }
 }
+
+// Repro from #28935
+
+type Foo = { tag: true, x: number } | { tag: false, y: number } | { [x: string]: string };
+
+function f30(foo: Foo) {
+    if (foo.tag) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}
+
+function f31(foo: Foo) {
+    if (foo.tag === true) {
+        foo;
+    }
+    else {
+        foo;
+    }
+}