diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ebe2d1c84b5..171b3182a54 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -19978,7 +19978,7 @@ namespace ts {
                 if (modifiersRelated) {
                     let result: Ternary;
                     const targetConstraint = getConstraintTypeFromMappedType(target);
-                    const sourceConstraint = instantiateType(getConstraintTypeFromMappedType(source), getCombinedMappedTypeOptionality(source) < 0 ? reportUnmeasurableMapper : reportUnreliableMapper);
+                    const sourceConstraint = instantiateType(getConstraintTypeFromMappedType(source), reportUnmeasurableMapper);
                     if (result = isRelatedTo(targetConstraint, sourceConstraint, RecursionFlags.Both, reportErrors)) {
                         const mapper = createTypeMapper([getTypeParameterFromMappedType(source)], [getTypeParameterFromMappedType(target)]);
                         if (instantiateType(getNameTypeFromMappedType(source), mapper) === instantiateType(getNameTypeFromMappedType(target), mapper)) {