diff --git a/app/scripts/dir_build b/app/scripts/dir_build
index 4a815f2ed1..c15b026531 100755
--- a/app/scripts/dir_build
+++ b/app/scripts/dir_build
@@ -76,10 +76,16 @@ if [[ $platform = "m" ]]; then
 else
 	# Windows / Linux: derive arch if not supplied
 	if [[ -z $arch ]]; then
-		case "$(uname -m)" in
-			arm64|aarch64) arch="arm64" ;;
-			x86_64)         arch="x64"   ;;
-		esac
+		# On Windows on ARM, an emulated x64 shell reports x86_64 from uname, so check
+		# the native architecture in the registry
+		if [[ $platform = "w" ]] && reg.exe query 'HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment' -v PROCESSOR_ARCHITECTURE 2> /dev/null | grep -q ARM64; then
+			arch="arm64"
+		else
+			case "$(uname -m)" in
+				arm64|aarch64) arch="arm64" ;;
+				x86_64)         arch="x64"   ;;
+			esac
+		fi
 	fi
 fi