diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index fd786799b8b4c..383566ab291c9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6832,11 +6832,9 @@ namespace ts {
                 }
             }
             else {
-                // First, if the constraint type is a type parameter, obtain the base constraint. Then,
-                // if the key type is a 'keyof X', obtain 'keyof C' where C is the base constraint of X.
-                // Finally, iterate over the constituents of the resulting iteration type.
-                const keyType = constraintType.flags & TypeFlags.InstantiableNonPrimitive ? getApparentType(constraintType) : constraintType;
-                const iterationType = keyType.flags & TypeFlags.Index ? getIndexType(getApparentType((<IndexType>keyType).type)) : keyType;
+                // If the key type is a 'keyof X', obtain 'keyof C' where C is the base constraint of X.
+                // Then iterate over the constituents of the key type.
+                const iterationType = constraintType.flags & TypeFlags.Index ? getIndexType(getApparentType((<IndexType>constraintType).type)) : constraintType;
                 forEachType(iterationType, addMemberForKeyType);
             }
             setStructuredTypeMembers(type, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
diff --git a/tests/baselines/reference/mappedTypeErrors.errors.txt b/tests/baselines/reference/mappedTypeErrors.errors.txt
index 6d78b229fa4e3..a59859875bddf 100644
--- a/tests/baselines/reference/mappedTypeErrors.errors.txt
+++ b/tests/baselines/reference/mappedTypeErrors.errors.txt
@@ -39,9 +39,11 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(130,39): error TS2322:
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(136,16): error TS2322: Type 'T' is not assignable to type 'string | number | symbol'.
   Type 'T' is not assignable to type 'symbol'.
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(136,21): error TS2536: Type 'P' cannot be used to index type 'T'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(148,17): error TS2339: Property 'foo' does not exist on type 'Pick<T, K>'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(152,17): error TS2339: Property 'foo' does not exist on type 'Record<K, number>'.
 
 
-==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (25 errors) ====
+==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (27 errors) ====
     interface Shape {
         name: string;
         width: number;
@@ -256,4 +258,18 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(136,21): error TS2536:
         pf: {x: 7},
         pt: {x: 7, y: false},
     };
+    
+    // Repro from #28170
+    
+    function test1<T, K extends keyof T>(obj: Pick<T, K>) {
+        let x = obj.foo;  // Error
+                    ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'Pick<T, K>'.
+    }
+    
+    function test2<T, K extends keyof T>(obj: Record<K, number>) {
+        let x = obj.foo;  // Error
+                    ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'Record<K, number>'.
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeErrors.js b/tests/baselines/reference/mappedTypeErrors.js
index bc73780a4787d..276aeb76a1d15 100644
--- a/tests/baselines/reference/mappedTypeErrors.js
+++ b/tests/baselines/reference/mappedTypeErrors.js
@@ -142,6 +142,16 @@ let f: Foo2<O, 'x'> = {
     pf: {x: 7},
     pt: {x: 7, y: false},
 };
+
+// Repro from #28170
+
+function test1<T, K extends keyof T>(obj: Pick<T, K>) {
+    let x = obj.foo;  // Error
+}
+
+function test2<T, K extends keyof T>(obj: Record<K, number>) {
+    let x = obj.foo;  // Error
+}
 
 
 //// [mappedTypeErrors.js]
@@ -222,6 +232,13 @@ var f = {
     pf: { x: 7 },
     pt: { x: 7, y: false }
 };
+// Repro from #28170
+function test1(obj) {
+    var x = obj.foo; // Error
+}
+function test2(obj) {
+    var x = obj.foo; // Error
+}
 
 
 //// [mappedTypeErrors.d.ts]
@@ -300,3 +317,5 @@ declare type O = {
 };
 declare let o: O;
 declare let f: Foo2<O, 'x'>;
+declare function test1<T, K extends keyof T>(obj: Pick<T, K>): void;
+declare function test2<T, K extends keyof T>(obj: Record<K, number>): void;
diff --git a/tests/baselines/reference/mappedTypeErrors.symbols b/tests/baselines/reference/mappedTypeErrors.symbols
index 80ce5d498459b..e2f3b57750606 100644
--- a/tests/baselines/reference/mappedTypeErrors.symbols
+++ b/tests/baselines/reference/mappedTypeErrors.symbols
@@ -540,3 +540,34 @@ let f: Foo2<O, 'x'> = {
 
 };
 
+// Repro from #28170
+
+function test1<T, K extends keyof T>(obj: Pick<T, K>) {
+>test1 : Symbol(test1, Decl(mappedTypeErrors.ts, 142, 2))
+>T : Symbol(T, Decl(mappedTypeErrors.ts, 146, 15))
+>K : Symbol(K, Decl(mappedTypeErrors.ts, 146, 17))
+>T : Symbol(T, Decl(mappedTypeErrors.ts, 146, 15))
+>obj : Symbol(obj, Decl(mappedTypeErrors.ts, 146, 37))
+>Pick : Symbol(Pick, Decl(lib.es5.d.ts, --, --))
+>T : Symbol(T, Decl(mappedTypeErrors.ts, 146, 15))
+>K : Symbol(K, Decl(mappedTypeErrors.ts, 146, 17))
+
+    let x = obj.foo;  // Error
+>x : Symbol(x, Decl(mappedTypeErrors.ts, 147, 7))
+>obj : Symbol(obj, Decl(mappedTypeErrors.ts, 146, 37))
+}
+
+function test2<T, K extends keyof T>(obj: Record<K, number>) {
+>test2 : Symbol(test2, Decl(mappedTypeErrors.ts, 148, 1))
+>T : Symbol(T, Decl(mappedTypeErrors.ts, 150, 15))
+>K : Symbol(K, Decl(mappedTypeErrors.ts, 150, 17))
+>T : Symbol(T, Decl(mappedTypeErrors.ts, 150, 15))
+>obj : Symbol(obj, Decl(mappedTypeErrors.ts, 150, 37))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+>K : Symbol(K, Decl(mappedTypeErrors.ts, 150, 17))
+
+    let x = obj.foo;  // Error
+>x : Symbol(x, Decl(mappedTypeErrors.ts, 151, 7))
+>obj : Symbol(obj, Decl(mappedTypeErrors.ts, 150, 37))
+}
+
diff --git a/tests/baselines/reference/mappedTypeErrors.types b/tests/baselines/reference/mappedTypeErrors.types
index c1ae739913798..cfa0df3cd586e 100644
--- a/tests/baselines/reference/mappedTypeErrors.types
+++ b/tests/baselines/reference/mappedTypeErrors.types
@@ -501,3 +501,27 @@ let f: Foo2<O, 'x'> = {
 
 };
 
+// Repro from #28170
+
+function test1<T, K extends keyof T>(obj: Pick<T, K>) {
+>test1 : <T, K extends keyof T>(obj: Pick<T, K>) => void
+>obj : Pick<T, K>
+
+    let x = obj.foo;  // Error
+>x : any
+>obj.foo : any
+>obj : Pick<T, K>
+>foo : any
+}
+
+function test2<T, K extends keyof T>(obj: Record<K, number>) {
+>test2 : <T, K extends keyof T>(obj: Record<K, number>) => void
+>obj : Record<K, number>
+
+    let x = obj.foo;  // Error
+>x : any
+>obj.foo : any
+>obj : Record<K, number>
+>foo : any
+}
+
diff --git a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
index cec1fbf839554..a38979054ecff 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeErrors.ts
@@ -144,3 +144,13 @@ let f: Foo2<O, 'x'> = {
     pf: {x: 7},
     pt: {x: 7, y: false},
 };
+
+// Repro from #28170
+
+function test1<T, K extends keyof T>(obj: Pick<T, K>) {
+    let x = obj.foo;  // Error
+}
+
+function test2<T, K extends keyof T>(obj: Record<K, number>) {
+    let x = obj.foo;  // Error
+}