diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 898ebbfbcd6ad..d1c0655e37a79 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12785,9 +12785,10 @@ namespace ts {
                             (<IndexedAccessType>template).indexType === getTypeParameterFromMappedType(target)) {
                             return Ternary.True;
                         }
+
                         if (!isGenericMappedType(source)) {
                             const targetConstraint = getConstraintTypeFromMappedType(target);
-                            const sourceKeys = getIndexType(source);
+                            const sourceKeys = filterType(getIndexType(source), t => !(t.flags & (TypeFlags.String | TypeFlags.Number)));
                             const hasOptionalUnionKeys = modifiers & MappedTypeModifiers.IncludeOptional && targetConstraint.flags & TypeFlags.Union;
                             const filteredByApplicability = hasOptionalUnionKeys ? filterType(targetConstraint, t => !!isRelatedTo(t, sourceKeys)) : undefined;
                             // A source type T is related to a target type { [P in Q]: X } if Q is related to keyof T and T[Q] is related to X.
diff --git a/tests/baselines/reference/indexSignatureAndMappedType.errors.txt b/tests/baselines/reference/indexSignatureAndMappedType.errors.txt
index 623fcd11bd050..9c1982eb49c69 100644
--- a/tests/baselines/reference/indexSignatureAndMappedType.errors.txt
+++ b/tests/baselines/reference/indexSignatureAndMappedType.errors.txt
@@ -2,9 +2,11 @@ tests/cases/compiler/indexSignatureAndMappedType.ts(6,5): error TS2322: Type '{
 tests/cases/compiler/indexSignatureAndMappedType.ts(15,5): error TS2322: Type 'Record<K, U>' is not assignable to type '{ [key: string]: T; }'.
   Type 'U' is not assignable to type 'T'.
 tests/cases/compiler/indexSignatureAndMappedType.ts(16,5): error TS2322: Type '{ [key: string]: T; }' is not assignable to type 'Record<K, U>'.
+tests/cases/compiler/indexSignatureAndMappedType.ts(37,5): error TS2322: Type '{ [x: string]: number; }' is not assignable to type 'Record<K, number>'.
+tests/cases/compiler/indexSignatureAndMappedType.ts(42,5): error TS2741: Property 'a' is missing in type '{ [x: string]: number; }' but required in type 'Record<"a", number>'.
 
 
-==== tests/cases/compiler/indexSignatureAndMappedType.ts (3 errors) ====
+==== tests/cases/compiler/indexSignatureAndMappedType.ts (5 errors) ====
     // A mapped type { [P in K]: X }, where K is a generic type, is related to
     // { [key: string]: Y } if X is related to Y.
     
@@ -44,4 +46,20 @@ tests/cases/compiler/indexSignatureAndMappedType.ts(16,5): error TS2322: Type '{
     interface IEntity<T extends string> extends IBaseEntity {
         properties: Record<T, string>;
     }
+    
+    // Repro from #28798
+    
+    function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
+        x = y; // error
+        ~
+!!! error TS2322: Type '{ [x: string]: number; }' is not assignable to type 'Record<K, number>'.
+        y = x;
+    }
+    
+    function concreteRecord(x: Record<'a', number>, y: { [x: string]: number }) {
+        x = y; // error
+        ~
+!!! error TS2741: Property 'a' is missing in type '{ [x: string]: number; }' but required in type 'Record<"a", number>'.
+        y = x;
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/indexSignatureAndMappedType.js b/tests/baselines/reference/indexSignatureAndMappedType.js
index be58286fb46cc..d7bb2a448edaa 100644
--- a/tests/baselines/reference/indexSignatureAndMappedType.js
+++ b/tests/baselines/reference/indexSignatureAndMappedType.js
@@ -31,6 +31,18 @@ interface IBaseEntity {
 interface IEntity<T extends string> extends IBaseEntity {
     properties: Record<T, string>;
 }
+
+// Repro from #28798
+
+function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
+    x = y; // error
+    y = x;
+}
+
+function concreteRecord(x: Record<'a', number>, y: { [x: string]: number }) {
+    x = y; // error
+    y = x;
+}
 
 
 //// [indexSignatureAndMappedType.js]
@@ -49,6 +61,15 @@ function f3(x, y) {
     x = y; // Error
     y = x; // Error
 }
+// Repro from #28798
+function constrainedRecord(x, y) {
+    x = y; // error
+    y = x;
+}
+function concreteRecord(x, y) {
+    x = y; // error
+    y = x;
+}
 
 
 //// [indexSignatureAndMappedType.d.ts]
@@ -71,3 +92,9 @@ interface IBaseEntity {
 interface IEntity<T extends string> extends IBaseEntity {
     properties: Record<T, string>;
 }
+declare function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: {
+    [x: string]: number;
+}): void;
+declare function concreteRecord(x: Record<'a', number>, y: {
+    [x: string]: number;
+}): void;
diff --git a/tests/baselines/reference/indexSignatureAndMappedType.symbols b/tests/baselines/reference/indexSignatureAndMappedType.symbols
index 0e33d65ee4028..2a1bb32925368 100644
--- a/tests/baselines/reference/indexSignatureAndMappedType.symbols
+++ b/tests/baselines/reference/indexSignatureAndMappedType.symbols
@@ -96,3 +96,39 @@ interface IEntity<T extends string> extends IBaseEntity {
 >T : Symbol(T, Decl(indexSignatureAndMappedType.ts, 29, 18))
 }
 
+// Repro from #28798
+
+function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
+>constrainedRecord : Symbol(constrainedRecord, Decl(indexSignatureAndMappedType.ts, 31, 1))
+>K : Symbol(K, Decl(indexSignatureAndMappedType.ts, 35, 27))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 35, 42))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+>K : Symbol(K, Decl(indexSignatureAndMappedType.ts, 35, 27))
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 35, 63))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 35, 70))
+
+    x = y; // error
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 35, 42))
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 35, 63))
+
+    y = x;
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 35, 63))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 35, 42))
+}
+
+function concreteRecord(x: Record<'a', number>, y: { [x: string]: number }) {
+>concreteRecord : Symbol(concreteRecord, Decl(indexSignatureAndMappedType.ts, 38, 1))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 40, 24))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 40, 47))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 40, 54))
+
+    x = y; // error
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 40, 24))
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 40, 47))
+
+    y = x;
+>y : Symbol(y, Decl(indexSignatureAndMappedType.ts, 40, 47))
+>x : Symbol(x, Decl(indexSignatureAndMappedType.ts, 40, 24))
+}
+
diff --git a/tests/baselines/reference/indexSignatureAndMappedType.types b/tests/baselines/reference/indexSignatureAndMappedType.types
index 9f3b3e3fe6cc5..cdc675f668c70 100644
--- a/tests/baselines/reference/indexSignatureAndMappedType.types
+++ b/tests/baselines/reference/indexSignatureAndMappedType.types
@@ -76,3 +76,39 @@ interface IEntity<T extends string> extends IBaseEntity {
 >properties : Record<T, string>
 }
 
+// Repro from #28798
+
+function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
+>constrainedRecord : <K extends "a">(x: Record<K, number>, y: { [x: string]: number; }) => void
+>x : Record<K, number>
+>y : { [x: string]: number; }
+>x : string
+
+    x = y; // error
+>x = y : { [x: string]: number; }
+>x : Record<K, number>
+>y : { [x: string]: number; }
+
+    y = x;
+>y = x : Record<K, number>
+>y : { [x: string]: number; }
+>x : Record<K, number>
+}
+
+function concreteRecord(x: Record<'a', number>, y: { [x: string]: number }) {
+>concreteRecord : (x: Record<"a", number>, y: { [x: string]: number; }) => void
+>x : Record<"a", number>
+>y : { [x: string]: number; }
+>x : string
+
+    x = y; // error
+>x = y : { [x: string]: number; }
+>x : Record<"a", number>
+>y : { [x: string]: number; }
+
+    y = x;
+>y = x : Record<"a", number>
+>y : { [x: string]: number; }
+>x : Record<"a", number>
+}
+
diff --git a/tests/cases/compiler/indexSignatureAndMappedType.ts b/tests/cases/compiler/indexSignatureAndMappedType.ts
index b5f9e8a0030ac..0dd665918ad5b 100644
--- a/tests/cases/compiler/indexSignatureAndMappedType.ts
+++ b/tests/cases/compiler/indexSignatureAndMappedType.ts
@@ -33,3 +33,15 @@ interface IBaseEntity {
 interface IEntity<T extends string> extends IBaseEntity {
     properties: Record<T, string>;
 }
+
+// Repro from #28798
+
+function constrainedRecord<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
+    x = y; // error
+    y = x;
+}
+
+function concreteRecord(x: Record<'a', number>, y: { [x: string]: number }) {
+    x = y; // error
+    y = x;
+}