diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index 034bb07b05bc..eae369889ce0 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -3596,8 +3596,8 @@ class ARMTargetInfo : public TargetInfo {
     DoubleAlign = LongLongAlign = LongDoubleAlign = SuitableAlign = 64;
     const llvm::Triple &T = getTriple();
 
-    // size_t is unsigned long on Darwin and NetBSD.
-    if (T.isOSDarwin() || T.getOS() == llvm::Triple::NetBSD)
+    // size_t is unsigned long on MachO-derived environments and NetBSD.
+    if (T.isOSBinFormatMachO() || T.getOS() == llvm::Triple::NetBSD)
       SizeType = UnsignedLong;
     else
       SizeType = UnsignedInt;
diff --git a/clang/test/Sema/arm-darwin-aapcs.cpp b/clang/test/Sema/arm-darwin-aapcs.cpp
index 1359a1dc15aa..3a6082ae40e2 100644
--- a/clang/test/Sema/arm-darwin-aapcs.cpp
+++ b/clang/test/Sema/arm-darwin-aapcs.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 %s -triple thumbv7-apple-ios -target-abi aapcs -verify -fsyntax-only
+// RUN: %clang_cc1 %s -triple thumbv7m-apple-macho -target-abi aapcs -verify -fsyntax-only
 // expected-no-diagnostics
 
 // ARM's AAPCS normally has size_t defined as unsigned int, but on Darwin