diff --git a/clang/include/clang/AST/CXXInheritance.h b/clang/include/clang/AST/CXXInheritance.h
index e89326081a180..72d365bfbc1f3 100644
--- a/clang/include/clang/AST/CXXInheritance.h
+++ b/clang/include/clang/AST/CXXInheritance.h
@@ -192,7 +192,7 @@ class CXXBasePaths {
   /// Determine whether the path from the most-derived type to the
   /// given base type is ambiguous (i.e., it refers to multiple subobjects of
   /// the same base type).
-  bool isAmbiguous(CanQualType BaseType);
+  bool isAmbiguous(CanQualType BaseType) const;
 
   /// Whether we are finding multiple paths to detect ambiguities.
   bool isFindingAmbiguities() const { return FindAmbiguities; }
diff --git a/clang/lib/AST/CXXInheritance.cpp b/clang/lib/AST/CXXInheritance.cpp
index 7a3e7ea4e5b8f..29f5916284ebb 100644
--- a/clang/lib/AST/CXXInheritance.cpp
+++ b/clang/lib/AST/CXXInheritance.cpp
@@ -34,9 +34,9 @@ using namespace clang;
 /// ambiguous, i.e., there are two or more paths that refer to
 /// different base class subobjects of the same type. BaseType must be
 /// an unqualified, canonical class type.
-bool CXXBasePaths::isAmbiguous(CanQualType BaseType) {
+bool CXXBasePaths::isAmbiguous(CanQualType BaseType) const {
   BaseType = BaseType.getUnqualifiedType();
-  IsVirtBaseAndNumberNonVirtBases Subobjects = ClassSubobjects[BaseType];
+  IsVirtBaseAndNumberNonVirtBases Subobjects = ClassSubobjects.lookup(BaseType);
   return Subobjects.NumberOfNonVirtBases + (Subobjects.IsVirtBase ? 1 : 0) > 1;
 }