diff -urpa rpm-4.5.orig/build/parsePreamble.c rpm-4.5/build/parsePreamble.c
--- rpm-4.5.orig/build/parsePreamble.c	2019-06-01 18:55:39.351000000 +0200
+++ rpm-4.5/build/parsePreamble.c	2019-06-01 19:24:30.398000000 +0200
@@ -730,19 +730,32 @@ static int handlePreambleTag(Spec spec,
     case RPMTAG_EXCLUSIVEOS:
 	addOrAppendListEntry(spec->sourceHeader, tag, field);
 	break;
-    case RPMTAG_BUILDARCHS:
-	if ((rc = poptParseArgvString(field,
-				      &(spec->BACount),
-				      &(spec->BANames)))) {
+    case RPMTAG_BUILDARCHS: {
+	int BACount;
+	const char **BANames = NULL;
+	if ((rc = poptParseArgvString(field, &BACount, &BANames))) {
 	    rpmError(RPMERR_BADSPEC,
 		     _("line %d: Bad BuildArchitecture format: %s\n"),
 		     spec->lineNum, spec->line);
 	    return RPMERR_BADSPEC;
 	}
-	if (!spec->BACount)
+	if (spec->packages == pkg) {
+	    spec->BACount = BACount;
+	    spec->BANames = BANames;
+	} else {
+	    if (BACount != 1 || strcmp(BANames[0], "noarch")) {
+		rpmError(RPMERR_BADSPEC,
+			_("line %d: Only noarch subpackages are supported: %s\n"),
+			spec->lineNum, spec->line);
+		BANames = _free(BANames);
+		return RPMRC_FAIL;
+	    }
+	    headerAddEntry(pkg->header, RPMTAG_ARCH, RPM_STRING_TYPE, "noarch", 1);
+	}
+	if (!BACount)
 	    spec->BANames = _free(spec->BANames);
 	break;
-
+    }
     default:
 	rpmError(RPMERR_INTERNAL, _("Internal error: Bogus tag %d\n"), tag);
 	return RPMERR_INTERNAL;
diff -urpa rpm-4.5.orig/build/parseSpec.c rpm-4.5/build/parseSpec.c
--- rpm-4.5.orig/build/parseSpec.c	2019-06-01 18:55:39.226000000 +0200
+++ rpm-4.5/build/parseSpec.c	2019-06-01 19:27:44.338000000 +0200
@@ -627,8 +627,11 @@ int parseSpec(rpmts ts, const char *spec
 	}
 
 	(void) headerAddEntry(pkg->header, RPMTAG_OS, RPM_STRING_TYPE, os, 1);
-	(void) headerAddEntry(pkg->header, RPMTAG_ARCH,
-		RPM_STRING_TYPE, arch, 1);
+	/* noarch subpackages already have arch set here, leave it alone */
+	if (!headerIsEntry(pkg->header, RPMTAG_ARCH)) {
+	    (void) headerAddEntry(pkg->header, RPMTAG_ARCH,
+		    RPM_STRING_TYPE, arch, 1);
+	}
 	(void) headerAddEntry(pkg->header, RPMTAG_PLATFORM,
 		RPM_STRING_TYPE, platform, 1);