diff --git a/lib/CppInterOp/CppInterOp.cpp b/lib/CppInterOp/CppInterOp.cpp
index 6ad4d8d7e..fb93e5dc0 100644
--- a/lib/CppInterOp/CppInterOp.cpp
+++ b/lib/CppInterOp/CppInterOp.cpp
@@ -878,12 +878,16 @@ TypeRef GetEnumConstantType(ConstDeclRef DRef) {
   return INTEROP_RETURN(nullptr);
 }
 
-size_t GetEnumConstantValue(ConstDeclRef DRef) {
+int64_t GetEnumConstantValue(ConstDeclRef DRef) {
   INTEROP_TRACE(DRef);
   const auto* D = unwrap<clang::Decl>(DRef);
   if (const auto* ECD = llvm::dyn_cast_or_null<clang::EnumConstantDecl>(D)) {
     const llvm::APSInt& Val = ECD->getInitVal();
-    return INTEROP_RETURN(Val.getExtValue());
+    if (Val.isRepresentableByInt64())
+      return INTEROP_RETURN(Val.getExtValue());
+    // Do not round-trip through a string and std::stoul: unsigned long is
+    // 32 bit on LLP64/ILP32 and throws out_of_range for these values.
+    return INTEROP_RETURN((int64_t)Val.getZExtValue());
   }
   return INTEROP_RETURN(0);
 }
diff --git a/lib/CppInterOp/CppInterOp.td b/lib/CppInterOp/CppInterOp.td
index be9cd1e86..4f4c45e86 100644
--- a/lib/CppInterOp/CppInterOp.td
+++ b/lib/CppInterOp/CppInterOp.td
@@ -1059,10 +1059,11 @@ def GetEnumConstantType : CppInterOpAPI {
 }
 
 def GetEnumConstantValue : CppInterOpAPI {
-  let Doc = [{Gets the index value (0,1,2, etcetera) of the enum constant
-that was passed into this function.}];
+  let Doc = [{Gets the value of the enum constant that was passed into this
+function, as its 64-bit bit pattern (values above INT64_MAX come back as
+their two's complement representation).}];
 
-  let ReturnType = "size_t";
+  let ReturnType = "int64_t";
   let Args = [
     Arg<"ConstDeclRef", "DRef">
   ];
diff --git a/unittests/CppInterOp/EnumReflectionTest.cpp b/unittests/CppInterOp/EnumReflectionTest.cpp
index 0e5d56025..0696cd71d 100644
--- a/unittests/CppInterOp/EnumReflectionTest.cpp
+++ b/unittests/CppInterOp/EnumReflectionTest.cpp
@@ -237,6 +237,10 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, EnumReflection_GetEnumConstantValue) {
       MinusTen = -10,
       MinusNine
     };
+    enum Huge : unsigned long long {
+      Big = ((unsigned long long)1) << 63,
+      Max = 0xFFFFFFFFFFFFFFFFULL
+    };
     int a = 10;
   )";
 
@@ -250,7 +254,12 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, EnumReflection_GetEnumConstantValue) {
   EXPECT_EQ(Cpp::GetEnumConstantValue(EnumConstants[4]), 54);
   EXPECT_EQ(Cpp::GetEnumConstantValue(EnumConstants[5]), -10);
   EXPECT_EQ(Cpp::GetEnumConstantValue(EnumConstants[6]), -9);
-  EXPECT_EQ(Cpp::GetEnumConstantValue(Decls[1]), 0); // Checking value of non enum constant
+  EXPECT_EQ(Cpp::GetEnumConstantValue(Decls[2]), 0); // Checking value of non enum constant
+
+  auto HugeConstants = Cpp::GetEnumConstants(Decls[1]);
+  EXPECT_EQ(Cpp::GetEnumConstantValue(HugeConstants[0]),
+            (int64_t)((uint64_t)1 << 63));
+  EXPECT_EQ(Cpp::GetEnumConstantValue(HugeConstants[1]), (int64_t)-1);
 }
 
 TYPED_TEST(CPPINTEROP_TEST_MODE, EnumReflection_GetEnums) {